refactor(coderd/x/chatd/chatdebug): delete the process-global step counters - #31008

Deletes chatdebug's process-global step counters: the stepCounters and runRefCounts maps, refCountMu, trackRunRef, releaseRunRef, nextStepNumber, syncStepCounter and CleanupStepCounter, the runtime.AddCleanup hook that ContextWithRun registered for every run context, and the two CleanupStepCounter calls in FinalizeRun. Since #31007 the database numbers steps, so no production code reads them.

Why it is safe: after #31007 no production code reads the counters. Production code only maintained them: ContextWithRun registered a reference for every run context and FinalizeRun called CleanupStepCounter, and this PR removes both. A search of coderd/ and enterprise/ for the deleted names finds nothing now. The deleted tests covered only the deleted code: TestNextStepNumber_Concurrent and TestSyncStepCounter_AdvancesCounter checked counter values, and the three garbage-collection tests in context_internal_test.go checked that a counter lives until the last RunContext for its run is collected. The 20 t.Cleanup(func() { CleanupStepCounter(runID) }) lines go with them. TestReuseStep_PreservesExistingHolder and TestBeginStepReuseStep stay. The FinalizeRun doc comment no longer mentions the counter.

Review focus: ContextWithRun is now a plain context.WithValue. Code that copies a RunContext, such as runnerDebugTurn.contextLocked, no longer affects anything, because no state is tied to the struct's lifetime.

Validation:

• go test -race -count=1 -p 4 -parallel 4 ./coderd/x/chatd/chatdebug/... passes.
• go test -count=1 -run Debug ./coderd/x/chatd/ (19 tests) passes, and the 25 chatd debug-run tests pass under -race (go test -race -run '^(TestActiveServer_ChatTurnDebugRun.*|TestRunnerDebugTurn.*|TestProposeChatTitle_DebugRun|TestArchiveChatDeletesDebugRuns|TestEditMessageDebugCleanup.*|TestQuickgen.*|TestMaybeGenerateChatTitle.*|TestPrepareManualTitleDebugRunCreateFailure)$' ./coderd/x/chatd/).
• golangci-lint run --fix=false ./coderd/x/chatd/chatdebug/... reports 0 issues, and the pre-commit hook passed.

Stack

This stack deletes the stale debug-run sweep, then simplifies the chatdebug debug-run, stream wrapper and step numbering code, drops the indexes the sweep used, and last deletes a runner debug field that lost its only reader with the heartbeats. The debug-run and stream wrapper refactors each sit on a test-only layer that pins the behavior they keep.

1. refactor(coderd): delete the chat debug run and step heartbeats #30988 Delete the chat debug run and step heartbeats
2. refactor: delete chatdebug FinalizeStale and the stale threshold #30989 Delete FinalizeStale and the stale threshold
3. refactor: delete the FinalizeStaleChatDebugRows query #30995 Delete the FinalizeStaleChatDebugRows query
4. 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
5. refactor(coderd/x/chatd): create and finalize debug runs through one helper #30934 Create and finalize debug runs through one helper
6. 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
7. refactor(coderd/x/chatd/chatdebug): share one finalize-once lifecycle between the stream wrappers #30981 Share one finalize-once lifecycle between the stream wrappers
8. refactor(coderd): number chat debug steps in the database #31007 Number chat debug steps in the database
9. This PR: Delete the process-global step counters
10. chore(coderd/database): drop the chat debug stale-sweep indexes #31009 Drop the chat debug stale-sweep indexes
11. refactor(coderd/x/chatd): drop the unused runnerCtx from the runner debug turn #31010 Drop the unused runnerCtx from the runner debug turn

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