refactor(coderd): contain the live chat stream in a Session module - #28755

Contains the live chat stream's orchestration in a self-contained Session module. The pubsub subscription, select loop, sync retry/backoff, sync poller registration, parts relay dialing and forwarding, and episode re-selection previously spanned eight stream_*.go files (about 1,680 lines) as unexported Server methods with no direct tests; races and reconnect bugs were only reachable through full WebSocket e2e tests.

Changes

• coderd/x/chatd/stream_session.go: NewSession(cfg) plus InitialSnapshot(), Events(), and Close(). Orchestration becomes hidden implementation behind that seam.
• The chat watch WebSocket handler in coderd/exp_chats.go constructs a Session from Server.StreamSessionConfig and forwards events.
• Direct unit tests drive the Session with in-memory pubsub and a dbmock store: subscribe/notify, reconnect and resubscribe, retry backoff, sync poller kicks, relay forwarding and episode re-selection, and Close teardown. Concurrency paths run with -race -count=3.
• Fixes a latent sync poller race where unregistering during an in-flight poll could send on a closed channel.
• Deletes the replaced stream_loop.go, stream_relay.go, stream_subscribe.go, stream_sync_poller.go, and their now-redundant internal tests; remaining repetitive cases are table-driven.
• ARCHITECTURE.md gains TODO(human) markers only where documented structure moved.

Net diff: 13 files, +1,494 / -1,601 (net -107 across production and tests). Behavior preserving: event ordering, retry/backoff semantics, and error classification are unchanged.

Validation

make gen, make fmt, make lint, pre-commit hooks, go test ./coderd/x/chatd/..., go test -race -count=3 on the session tests, and the TestStreamChat e2e smoke all pass. Remote dogfood UAT passed: incremental convergence after mid-stream refresh, interrupt, two-tab consistency, subagent child streams, and idle snapshot reopening, with zero duplicated or missing parts at the protocol level. A silent-then-burst delivery pattern observed during UAT was A/B tested against the parent commit and reproduces identically on main, so it is pre-existing environmental behavior, not owned by this change.

Disclosure: Xum (an AI coding agent) authored this PR on behalf of @ibetitsmike.