refactor(coderd/x/chatd): set the chatdebug stale threshold at construction - #30932

chatd sets the debug stale threshold once, in debugSvcFactory, before any caller can reach the service. This PR replaces Service.SetStaleAfter with a WithStaleAfter option and deletes the code that let a running heartbeat pick up a later change: the atomic threshold, the mutex, the wake-up channel, and the resets_ticker_on_threshold_change subtest that tested them. The step and run heartbeat launchers now share one loop, and the test-only WithClock option is gone.

It also deletes a chatd.go comment that said debug runs do not heartbeat during model streams. Both launchers do.

Why it is safe: SetStaleAfter had one production caller, in debugSvcFactory, which ran before the factory returned the service. A repo-wide search finds no reference to SetStaleAfter, thresholdChan, staleThreshold, staleAfterNanos or the chatdebug WithClock. TestLaunchHeartbeat (#30931) and TestService_FinalizeStale_UsesConfiguredThreshold cover the new code. The second is now a table that also checks that a zero or negative WithStaleAfter keeps DefaultStaleThreshold. All 13 mutants of the option, the shared loop, both launchers and FinalizeStale fail these tests, including ones that make WithStaleAfter accept zero or negative values.

Review focus: the shared heartbeat loop in service.go. The one visible change is a log message: run heartbeat failures now log heartbeat touch failed, the message step heartbeats already used, instead of run heartbeat touch failed.

Validation:

• go test -race -count=1 -p 4 -parallel 4 ./coderd/x/chatd/chatdebug/... passes before and after.
• go test -race -count=1 -p 4 -parallel 4 ./coderd/x/chatd/ -run '^(TestActiveServer_ChatTurnDebugRun.*|TestRunnerDebugTurn.*|TestProposeChatTitle_DebugRun|TestArchiveChatDeletesDebugRuns|TestEditMessageDebugCleanup.*)$': the same 16 tests pass before and after.
• golangci-lint run --fix=false ./coderd/x/chatd/chatdebug/... ./coderd/x/chatd/ reports 0 issues, and the pre-commit hook passed.

Stack

This stack simplifies the chatdebug heartbeat, debug-run and stream wrapper code. Each refactor sits on a test-only layer that pins the behavior it keeps.

1. test(coderd/x/chatd/chatdebug): cover both debug heartbeat launchers #30931 Cover both debug heartbeat launchers
2. refactor(coderd/x/chatd): set the chatdebug stale threshold at construction #30932 Set the chatdebug stale threshold at construction
3. test(coderd/x/chatd): pin the debug runs that turns, titles and status labels record #30933 Pin the debug runs that turns, titles and status labels record
4. refactor(coderd/x/chatd): create and finalize debug runs through one helper #30934 Create and finalize debug runs through one helper
5. test(coderd/x/chatd/chatdebug): table the stream wrapper tests and cover their status rules #30980 Table the stream wrapper tests and cover their status rules
6. refactor(coderd/x/chatd/chatdebug): share one finalize-once lifecycle between the stream wrappers #30981 Share one finalize-once lifecycle between the stream wrappers

Xum, an AI agent, opened this PR for @ibetitsmike and wrote this description.