fix(cli): expose /run_sse event framing as a public google.adk.cli.sse module - #7594

Summary

/run_sse owns a wire format ADK clients parse: internal metadata dropped, credential secrets redacted, and an event carrying an artifactDelta split into a content frame plus a content-less action frame. That logic lived inline in event_generator() in cli/api_server.py and depended on two private helpers (public_event, _redact_credential_secrets), so any application running a Runner outside the HTTP handler had to copy it and re-sync it by hand on every release. The framing changed in 2.11 (public_event entered the stream) and again in 2.12 (credential redaction), and each change silently broke downstream copies.

This extracts the framing into a public module and makes run_agent_sse() use it, so the export and the built-in server cannot diverge.

Fixes #7578. This is item 1 of that issue only. Item 2 (an optional SSE keep-alive) is intentionally left out: it adds a configurable timer to /run_sse and is a separate behavior change, as the issue itself proposes splitting them into two PRs.

Changes

• New src/google/adk/cli/sse.py
  • serialize_event(event) -> str: the JSON text of a data: frame. Applies public_event, model_dump(exclude_none=True, by_alias=True, mode="json"), and _redact_credential_secrets, in that order.
  • format_event_frames(event, *, split_artifact_delta=True) -> list[str]: the one-or-two data: <json>\n\n frames /run_sse writes, with the artifactDelta split.
• src/google/adk/cli/api_server.py: the inline block in event_generator() is replaced by format_event_frames(...). The existing function-call-resume guard is preserved as split_artifact_delta=not req.function_call_event_id, so resume requests still emit a single unsplit frame.

No behavior change: the pipeline, field masking, redaction order, and split condition are identical to before.

Tests

tests/unittests/cli/test_sse.py (new, 7 tests) covers wire aliases and None-stripping, credential redaction, internal-metadata removal, the artifactDelta split, split_artifact_delta=False, a content-less artifactDelta not being split, and that formatting does not mutate the caller's event.

tests/unittests/cli/test_fast_api.py gains test_agent_run_sse_frames_match_the_public_serializer, which asserts response.text == "".join(format_event_frames(event)). This is the guard the issue asks for: a framing change made in the route but not in the helper fails that test.

Testing plan

$ .venv/bin/python -m pytest tests/unittests/cli/test_sse.py -q
7 passed, 1 warning in 0.57s

$ .venv/bin/python -m pytest tests/unittests/cli -q
1497 passed, 5 skipped, 1 xfailed in 98.93s

The pre-existing /run_sse framing tests pass unchanged, which is the evidence that the extracted helper produces byte-identical output.

Notes for reviewers

• The public surface is two functions in a new google.adk.cli.sse module. The contract was already de facto public (clients parse it); this makes it explicit. Happy to rename or move it if you prefer a different home.
• serialize_event deliberately keeps calling the existing private _redact_credential_secrets rather than duplicating redaction logic. If that helper should also become public, that is a separate small change.