refactor(coderd/x/chatd): drop the unused runnerCtx from the runner debug turn - #31010

Deletes runnerDebugTurn.runnerCtx and the context parameter of newRunnerDebugTurn. The field's only reader was the LaunchRunHeartbeat call in ensureLocked, which #30988 deleted with the run heartbeats, so nothing has read it since.

Why it is safe: the constructor set the field and no code read it. The debug turn still uses the context of each call to Ensure, Context and Finalize, which this PR does not change. runner.go and 10 tests stop passing a context, and four of those tests drop the context.WithCancel that they created only for this parameter.

Validation:

• go build ./coderd/x/chatd/... and go vet ./coderd/x/chatd/... pass.
• The 25 chatd debug-run tests, TestRecordGenerationFinishFailure, TestSessionStartDispatchFailureFinishesGeneration and TestStartGeneration_CancelsDeletedToolCallsOnAgent pass under -race.
• golangci-lint run --fix=false ./coderd/x/chatd/ 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. refactor(coderd/x/chatd/chatdebug): delete the process-global step counters #31008 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. This PR: Drop the unused runnerCtx from the runner debug turn

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