test(coderd): share one server across the chat message pagination tests - #30965

TestGetChatMessages_Pagination started a coderd server with its own database in each of its ten leaves, although the leaves differ only in cursors and their own chat rows. This PR builds the client, database, owner, and chat model once in the parent. The six successful-page leaves become one table (options, expected ID positions, HasMore, queued count), and the two malformed after_id leaves become another. The transposed or equal cursor case and the burst walk stay separate subtests, and every subtest keeps its name.

Why it is safe: every leaf still creates its own chat, messages, and queued message, and every request and query is scoped to that chat. No leaf changes users, roles, models, or deployment values. I ran 24 mutants of getChatMessages and its two paginated queries (limit+1 and HasMore, truncation, ASC dispatch, queued suppression, after_id parsing, the transposed check, cursor comparison, and ordering) against the old and the new test with go test -overlay. The new test fails on all 20 mutants that the old test fails on, in the same leaves. It also fails on the two mutants that drop the chat_id filter, which the old test missed because each leaf had its own database. Two mutants survive both tests: the DESC path fetching limit rows instead of limit+1, and HasMore using >=. No test on main covers them, and this PR does not add coverage.

Review focus: the shared database interleaves message IDs across leaves. Assertions use only the IDs that each leaf's seedChat returned, so interleaving cannot change a result for a correct handler. AfterIDAtOrAboveMaxReturnsEmpty now seeds five messages instead of three, like the other page cases, and still sets after_id to the last ID.

Timing (prebuilt test binaries, -test.parallel 4, median of 5 alternating runs on a shared host): binary wall time went from 1.21 s to 0.72 s. Leaf times summed went from 3.89 s to 0.39 s, plus 0.48 s for the one shared server, and coderd boots went from 10 to 1.

Validation:

• go test ./coderd -run '^TestGetChatMessages_Pagination$' -count=10: pass
• the same run with -race -count=2: pass
• go vet ./coderd and golangci-lint run ./coderd/...: clean; the pre-commit hook passed

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