chore: delete dead functions and skip-only or duplicated tests - #29461

Three mechanical clean-ups from the test-suite audit, one commit each:

1. Skip-only stub tests. Six tests in coderd/exp_chats_test.go whose whole body sat behind an unconditional t.Skip ("legacy chat provider API removed"), plus TestSuspendedPagination (its comment says it existed to check db-fake parity, and dbmem is gone) and TestWorkspaceFilter (its comment says TestWorkspaceFilterManual covers it). The other eight skip-only tests stay: their skip reasons (flakes tracked in issues, manual tests, a pending upstream refactor) still hold.
2. Duplicated tests. TestServer/Logging/{CreatesFile,Human,JSON} started three server processes to assert that a log file appears (Human duplicated CreatesFile); they are now one server with a {flag, file} table. TestServer_Logging_NoParallel/{Stackdriver,Multiple} are one server with three sinks. The four aibridge/provider Test*_TypeAndName tests are one table.
3. Functions unreachable from any test or binary. golang.org/x/tools/cmd/deadcode -tags testsmallbatch ./cmd/... ./enterprise/cmd/... with and without -test; the intersection (dead even with every test as a root) is deleted, including the cascade (noopContainerCLI, NoopResolver, NoopResponseWriter, StatusQueue, five unused dbgen helpers, unused functional options, and so on). Left alone on purpose: scripts/rules.go (ruleguard DSL), vpn/ (live on darwin/windows), the coderd/debug.go _debug* swagger stubs, cli.SlimUnsupported (used under the slim build tag), and exported codersdk/agentsdk/workspacesdk symbols that need an external-consumer check first (ChatMessageSource, IsBedrockConfigured, IsChatGitWatchFallbackMessage, OAuth2RevocationTokenTypeHint.Valid, ParameterResolver.Resolve, ValidateNewWorkspaceParameters, ValidateWorkspaceBuildParameters, WithTrace, ServiceBannerFromProto, Stats.SessionCount, AgentReconnectingPTYInitWithContainer).

Net: 46 files, +160 / -2285 lines (582 lines of production code, the rest tests).

Validation

• go build ./... and go vet -tags=testsmallbatch ./... (compiles every test; only two pre-existing copylocks findings in untouched files)
• golangci-lint run over all 31 touched packages with a fresh cache: 0 issues
• deadcode rerun after the deletions: no new cascade; only the deliberately skipped symbols remain
• go test -race -count=2 -shuffle=on on the folded tests (./cli/ -run 'TestServer/Logging|TestServer_Logging_NoParallel', ./aibridge/provider/): pass
• Repository pre-commit hook passed on each commit

Part of the test-suite speedup series (see #29457); independent of the other PRs.

Remote validation

Remote run on a dogfood workspace (separate compute), chat https://dogfood.cdr.dev/agents/5bbe73d0-866a-4841-9cc3-c5e5b4d97e44, tested head 3821cba7af8: go build ./... and go vet -tags=testsmallbatch ./... (only the two pre-existing lock-copy findings in untouched files), cli logging tests and aibridge/provider pass, git grep confirms only the intentionally kept codersdk symbol remains; the runner independently verified on base that all eight deleted non-folded tests began with t.Skip. Endorsed verdict: PASS.

Xum (an AI agent) authored this PR on behalf of @ibetitsmike.