refactor(coderd): number chat debug steps in the database - #31007

InsertChatDebugStep now numbers a step after the run's highest step when the caller passes zero, so chatdebug stops numbering steps in process memory. CreateStepParams loses StepNumber, beginStep no longer calls nextStepNumber or syncStepCounter, and CreateStep replaces its collision retry, which listed the run's steps and bumped the number, with a per-run lock. #31008 deletes the counters, which no production code reads after this PR.

Why it is safe: Postgres takes the insert's snapshot before the statement waits on the run row, so creates that waited on the same run read the same highest step. CreateStep therefore takes pg_advisory_xact_lock(LockIDChatDebugRunSteps(run)) in its own statement and inserts in the same transaction, so each insert starts after the previous holder committed. The new TestService_CreateStep_NumbersConcurrentSteps holds the run row, starts 20 creates, waits until all of them wait on a lock, then commits and checks the numbers 1 to 20. It passes 10 of 10 runs, and fails 10 of 10 with a 10-attempt collision retry in place of the lock. TestService_CreateStep_RetriesDuplicateStepNumbers becomes TestService_CreateStep_NumbersStepsPerRun, and TestService_CreateStep_ListRetryErrorWins goes with the list call it covered. Mutants that fail the chatdebug tests: no lock, the lock outside the transaction, the lock keyed by chat, a constant number, storing the zero, numbering across runs, a nonzero number from CreateStep, and an unclassified missing run. One mutant survives and is equivalent: running the insert on s.db instead of tx inside the same InTx callback, because the insert still commits before the transaction releases the lock. Running the insert after the transaction commits fails the new test with a duplicate key. A copy of service.go with garbage in it fails to build.

Review focus: @step_number stays as an override. Production passes zero, and the fixtures in querier_test.go, dbauthz_test.go, dbpurge_test.go and exp_chats_test.go keep their explicit numbers. No test depends on the override (a mutant that ignores it survives), because every fixture numbers its steps in insert order. Dropping the parameter would touch about 20 fixtures outside chatdebug.

The PR removes more than it adds: +179/-233 hand-written lines. The generated querier.go and queries.sql.go change 15 more.

Validation:

• make gen/db changes only querier.go and queries.sql.go.
• go test -race -count=1 -p 4 -parallel 4 ./coderd/x/chatd/chatdebug/... passes, and go test -count=10 -run TestService_CreateStep ./coderd/x/chatd/chatdebug/ passes all 6 tests 10 of 10 times.
• go test -count=1 -run ChatDebug ./coderd/database/ ./coderd/database/dbpurge/ and go test -count=1 -run Debug ./coderd/x/chatd/ (19 tests) pass, and the 25 chatd debug-run tests pass under -race.
• golangci-lint run --fix=false ./coderd/x/chatd/chatdebug/... ./coderd/database/ 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. This PR: 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. 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.