refactor(coderd/x/chatd): create and finalize debug runs through one helper - #30934

Chat turns, manual titles, automatic titles and turn status labels each created a debug run on a detached, time-bounded context, rebuilt the run context from the row, and finalized the run by hand. This PR adds chatdebug.Service.BeginRun and StartRun and routes all four through them. It deletes prepareQuickgenDebugCandidate, shortTextCandidate, chatdebugRunContext, debugCreateRunTimeout and a literal 5 second insert timeout.

• BeginRun inserts the run on a context that ignores the caller's cancellation, with a 5 second limit, and builds the RunContext from its parameters. Chat turns call it and still finalize in runnerDebugTurn.
• StartRun wraps BeginRun for one model call and returns a context that carries the run plus a function that finalizes it from the call's error. Titles and status labels use it.
• New TestService_StartRun covers a finalized run, a failed insert and a failed finalize with a mock store, including the warning each failure logs.

Why it is safe: #30933 pins the insert parameters, the run context, the finalize status and summary, and the failed insert for all four call sites. Those tests and the other debug-run, title and quickgen tests pass before and after. CreateRun stores its parameters unchanged, so a RunContext built from the parameters equals one built from the row. A repo-wide search finds no reference to the deleted symbols. Of 13 mutants of BeginRun and StartRun, 12 fail TestService_StartRun; the survivor shortens the insert limit from 5 seconds to 1.

Review focus: log output. Create and finalize failures now log through the chatdebug logger as failed to create debug run and failed to finalize debug run. Before, each call site used its own logger and message. Compaction still creates its debug run by hand, because #30766 changes that code; it can move to StartRun after that PR merges.

Validation:

• go test -race -count=1 -p 4 -parallel 4 ./coderd/x/chatd/ -run '^(TestActiveServer_ChatTurnDebugRun.*|TestRunnerDebugTurn.*|TestProposeChatTitle_DebugRun|TestArchiveChatDeletesDebugRuns|TestEditMessageDebugCleanup.*|TestQuickgen.*|TestMaybeGenerateChatTitle.*|TestPrepareManualTitleDebugRunCreateFailure)$': the same 25 tests pass before and after.
• go test -race -count=1 -p 4 -parallel 4 ./coderd/x/chatd/chatdebug/... passes.
• 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.