Fix LiteRT worker request leaks and fail closed on interrupted cleanup

[leehack]

Summary

When a LiteRT-LM worker stopped during an outstanding request, the caller's response port and future could remain alive after CLI run_end. Track and fail pending requests on worker termination, close their ports, observe unexpected worker exits/errors, and share concurrent disposal results. Preserve unverified-cleanup failures through free retries, cancellation races, and automatic backend routing instead of reporting success or replacing the failed runtime.

Partially addresses #521. Native initialization latency remains a separate limitation: one preserved macOS Qwen run exceeded the unchanged 60-second limit, while subsequent runs completed in 48–52 seconds. No native runtime, pin, cache policy, or timeout threshold changes. Requested GPU placement is unverified.

Validation on 97223924ee9a02d61f19965c294c0665b847c066 / base 9946295a77728f2c73e9f8454225f06eddbad122:

• VM: 2,158 passed / 77 skipped; Chrome: 945 passed; analysis, pinned formatting, docs build and links passed.
• Affected focused suite: 241 passed. Independent review: 139 passing test cases, zero known change-caused P1/P2 findings; review findings fixed and re-reviewed. Independent Chrome launch was sandbox-blocked; primary Chrome passed.
• Mutation checks rejected removed pending-request tracking and worker-exit observation.
• Qwen3.5 0.8B INT8 / LiteRT 0.17.0-5 / macOS arm64: final-head initial/reload/missing-model recovery hello 49.680/49.654/50.060s; cleanup and natural exit passed. Forced initialization timeout reported cleanup error and pending failure, then exited naturally within its outer deadline. Prior 60s failure retained as a failure.
• Runtime archive SHA256 56760ac7d4678b9cada5041b7a2ee327183d41861a64152dead8338a2392f8cd; model SHA256 684d4d34adf7176eb47f6026ff65c33d42584737254e5524a8d1ad62edc21b98, revision c23b16e43ada6ead533b12593fe500bbe268014f.

High-risk surfaces: backendRuntime, regressionPolicy. Required independent exact-head review completed locally. PR-bound evaluator evidence is populated for PR #527 and the exact head/base below. The evaluator reports unverifiedPrerequisites: local evidence is internally consistent, but authenticated auditor/App/environment/ruleset prerequisites are unavailable. All 16 hosted checks passed on this exact head; live review threads total zero. No external authenticated readiness claim is made here. Other devices/platforms were not real-model tested for this change. Blocking native FFI calls still require an outer process deadline in CLI validation.

Production-readiness scope

• User-facing scope: Pending LiteRT requests fail when their worker stops; unverified native cleanup cannot report success or be bypassed through the same router.
• Supported platforms/paths: Native LiteRT worker and automatic backend routing; macOS arm64 real-model evidence, platform-neutral VM regression coverage.
• Unsupported or unavailable paths: Blocking native FFI termination is not guaranteed; CLI callers need an outer process deadline. Failed cleanup throws typed errors and requires process restart.
• Out of scope / follow-ups: Native reload latency remains in Qwen3.5 LiteRT GPU reload times out on macOS and leaves CLI alive #521; no cache/runtime/pin changes or GPU-placement qualification.

Completeness checklist

• Scope explicitly reduced to demonstrated Dart lifecycle cleanup.
• Unverified cleanup fails loudly with actionable typed errors.
• Lifecycle docs, test matrix, and changelogs updated.
• Positive reload/recovery and negative worker/cleanup regression coverage added.
• New production errors do not include secret-bearing paths or native exception details.
• Remaining native latency investigation stays tracked in Qwen3.5 LiteRT GPU reload times out on macOS and leaves CLI alive #521.

Test Plan

• dart run tool/prepare_workspace.dart
• dart format --output=none --set-exit-if-changed .
• dart analyze
• dart test -p vm -j 1 --exclude-tags local-only --coverage=.dart_tool/issue521/coverage-refresh-9946295
• dart test -p chrome --exclude-tags local-only
• Docs build/link validation; targeted backend/router tests; real-model lifecycle scenario.

Matrix Evidence

Matrix row	Scope	Platform / model / backend	Result	Evidence
static-format-analyze	Static gates	Flutter 3.47.1	PASS	Prepared workspace, format/analyze/boundaries
root-vm	Unit/integration	macOS VM	PASS	2158 passed, 77 skipped
root-chrome	Browser compatibility	Chrome	PASS	945 passed
coverage-lib	Library line coverage	VM	PASS	80.52% (13585/16871); unchanged 70% threshold
litert-lm-lifecycle	Reload/recovery and timeout cleanup	Qwen3.5 INT8 / LiteRT 0.17.0-5 / macOS arm64 / GPU requested	PASS on final head	3 hello cycles plus forced-timeout child; prior 60s failure retained
high-risk-exact-head-independent-qa	Blocking review	Exact head/base below	PASS locally	Fresh independent reviewer, 139 tests; 0 P1/P2

High-risk regression review

• Classification: high-risk; backendRuntime, regressionPolicy.
• Implementation task: 01a0b507-c270-7141-b98c-840c72ecca55.
• Independent blocking QA task: fresh subagent /root/audit_521.
• Exact head / current base: 97223924ee9a02d61f19965c294c0665b847c066 / 9946295a77728f2c73e9f8454225f06eddbad122.
• Production-branch deletion/bypass proof: Tests failed when pending-request tracking or worker-exit observation was removed; retry/routing/cancellation regressions directly invoke production paths.
• Affected-family evidence: Qwen3.5 0.8B INT8 and published LiteRT 0.17.0-5 as identified above; final bounded recovery and timeout cleanup passed.
• Unavailable evidence: Other native platforms/devices; verified GPU placement; authenticated external readiness prerequisites.
• Known change-caused P1 regressions: 0.
• Unresolved review threads: 0 at PR creation; recheck live before mark-ready/merge.

Review Notes

Independent exact-head review accepted with zero P1/P2 findings. This remains a narrow cleanup fix; #521 is not closed by it. All hosted CI checks passed on the real PR head. This PR remains draft; no mark-ready or merge authorization.

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