Phase 5.2 of the PG Queue rollout (epic UN-3445). Adds fairness-header
support to the third dispatch path (sdk1's ExecutionDispatcher) so
``execute_extraction`` tasks emitted by file_processing carry the same
routing metadata as workflow-execution dispatches that go through
queue_backend.

What

* sdk1/execution/dispatcher.py: ``dispatch``, ``dispatch_async``,
  ``dispatch_with_callback`` all accept an optional ``headers`` kwarg.
  When non-None, forwarded to Celery's send_task; when None, omitted
  so the call shape stays identical to pre-Phase-5.2 for callers that
  don't opt in (sdk1's existing tests remain green unchanged).
* queue_backend/fairness.py: new ``FairnessKey.as_header()`` method
  returns the wire-ready ``{"x-fairness-key": ...}`` dict. Producers
  no longer need to reference ``FAIRNESS_HEADER_NAME`` directly —
  keeps the additive-only canary in test_fairness_key.py happy.
* file_processing/structure_tool_task.py: small ``_fairness_headers``
  helper builds the header (defaulting workload_type to NON_API;
  propagating the real type is Phase 6 work). All three
  ``dispatcher.dispatch(...)`` sites (lines 468, 507, 720) now pass
  ``headers=_fairness_headers(organization_id)``.
* tests/test_executor_dispatch.py: new file. Covers header forwarding
  through all three dispatcher methods (including the "omit when
  None" pre-existing shape preservation), the FairnessKey.as_header()
  shape, and an AST inventory canary that forbids raw
  ``*.send_task("execute_extraction", ...)`` outside
  ExecutionDispatcher.

Why

UN-3501 plumbed fairness on bare dispatch() call sites. The
``execute_extraction`` task is the most workflow-execution-y dispatch
in the codebase but bypasses queue_backend (uses ExecutionDispatcher
directly), so it had no fairness header. The canary in
test_fairness_key.py audits only bare-name dispatch() and missed it.

No regression risk

* Additive: ``headers`` is optional and defaults to None on all three
  dispatcher methods; the existing 78 sdk1 tests pass unchanged.
* Producer-side only — no consumer reads ``x-fairness-key`` yet.
* No queue routing, task name, or args/kwargs change.

Test count: workers seam suite 53 -> 60 (new test_executor_dispatch.py
with 7 tests). sdk1 dispatcher suite 80/80 green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

…n a list'

Greptile P2: function builds and returns a list — it is not a
generator — but the docstring opened with 'Yield ...', which would
mislead a reader into expecting lazy consumption / generator semantics
(early break, send(), etc.).

Pure docstring fix, no behaviour change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

…test-infra fix)

Seven of Vishnu's findings against ``524ae9184`` addressed. Three
flagged IMPORTANT (silent-failure + missing test coverage), four
SUGGESTION (drift hazard, comment/behaviour mismatch, hollow canary,
duplicate-test cleanup). The other three (hand-built fixture,
SDK1 ``dict[str, Any]`` boundary, ``as_header`` TypedDict refactor)
deferred — see PR thread acknowledgments.

* **#11a (SUGGESTION, drift)** — ``workers/queue_backend/dispatch.py``
  still hand-built the fairness header instead of calling
  ``fairness.as_header()``. Wire-format encoding now has a single
  source so the two sites can't drift. ``FAIRNESS_HEADER_NAME``
  import dropped (no longer used here).

* **#9 (SUGGESTION, comment/behaviour mismatch)** —
  ``ExecutionDispatcher._build_send_kwargs`` was forwarding ``{}``
  as-is despite the docstring calling it "likely indicates a
  producer-side build bug". Changed ``if headers is not None`` ->
  ``if headers``: falsy is dropped so the on-wire shape matches the
  no-headers baseline and a miswired producer surfaces immediately.
  Docstring rewritten to describe the new contract.

* **#3 (SUGGESTION, wording)** — ``canary_helpers`` docstring
  overstated adoption. Softened to "intended single home" and
  named the two characterisation walkers still inlining the logic
  as a known follow-up.

* **#5 + #6 (IMPORTANT cleanup + SUGGESTION fixture)** — six
  structurally-identical ``*_forwards_headers`` /
  ``*_omits_headers_when_none`` tests collapsed to three
  parametrized methods over the three dispatch entry points.
  Fixture now uses ``FairnessKey(...).as_header()`` rather than
  hand-built dicts, so the wire shape exercised matches what real
  producers emit (including ``pipeline_priority``). Net: ~60 LOC
  removed, per-method failure granularity preserved via parametrize
  IDs. Also added empty-dict drop assertions covering #9.

* **#7 (SUGGESTION, missing combined test)** — new
  ``test_dispatch_with_callback_combines_headers_and_callbacks``
  passes ``on_success``, ``on_error``, ``task_id``, and ``headers``
  together and asserts all four land on the same ``send_task``
  call. A key-merge regression in ``_build_send_kwargs`` would
  have slipped through the single-kwarg forwarding tests.

* **#8 (SUGGESTION, hollow canary)** — the
  ``execute_extraction`` dispatch canary only ever asserted the
  empty (passing) case against the live tree. Added a positive-
  detection unit test feeding ``ast.parse`` of a known-bad snippet
  and a blind-spot lock test (constant ref, f-string,
  ``apply_async`` all evade the detector — documenting the scope
  so a future widening intentionally trips the asserts).

* **#4 (IMPORTANT, untested helper)** — ``_fairness_headers`` in
  ``structure_tool_task`` was untested; a regression flipping
  ``NON_API`` -> ``API`` or dropping ``headers=`` at any of the
  three call sites would have stayed green. Added focused unit
  tests in new ``test_structure_tool_task.py`` (wire shape,
  org_id propagation, ``NON_API`` not ``API``) and extended
  ``test_sanity_phase5.TestStructureToolSingleDispatch`` to assert
  ``dispatch.call_args.kwargs["headers"]`` carries the expected
  shape.

* **#2 (IMPORTANT, vacuous-pass)** — ``iter_production_trees``
  warned-and-continued on ``SyntaxError`` but neither canary
  module promoted the warning to error. A botched merge in a
  production file would have dropped silently from the audit set
  and every canary would have passed vacuously over a smaller
  tree. Added ``pytestmark = pytest.mark.filterwarnings(
  "error::UserWarning")`` on both ``test_executor_dispatch`` and
  ``test_fairness_key``, plus a new ``test_canary_helpers.py``
  that unit-tests both the warn-on-broken behaviour and the
  promote-to-error contract the canary modules rely on.

**Bundled test-infra fix (unrelated but unblocks CI):** the
``test_callback_sanity.TestEagerHealthcheckRoundTrip`` tests
selected the healthcheck task via ``endswith(".healthcheck")``
against ``eager_app.tasks``, which is a shared celery global
registry containing ``callback.worker.healthcheck``,
``executor.worker.healthcheck``,
``file_processing.worker.healthcheck`` etc. The bare ``next(...)``
returned whichever was inserted first — non-deterministic across
pytest module-collection orders. Without this fix, the new tests
added in this commit perturb the collection profile enough to
flip the failure rate from ~10% to nearly 100%. Replaced with
exact-name lookup ``name == "callback.worker.healthcheck"``.
Identical fix already landed on the UN-3513 branch (see #2020).

Test count: 31 -> 42 on the UN-3508-touched modules. Full workers
suite: 6 failures pre-existing baseline, unchanged by this commit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>