fix(frontend): handle orphaned api cleanup promise rejection

[dborup]

Summary

public/app.js's api() helper does:

_inflight.set(path, promise);
promise.finally(() => _inflight.delete(path));
return promise;

.finally() returns its own derived promise that mirrors the outcome of the promise it's attached to. That derived promise was discarded uncaught, so when a request rejects, both the promise returned to callers (correctly awaited/caught by every real caller) and the orphaned .finally() promise reject — the second one with nothing ever attached to observe it. In a browser this only shows up as a benign Uncaught (in promise) console warning; it surfaced here because Node's stricter default (an unhandled rejection crashes the process) has no browser equivalent, while building a behavioral test for issue Kpa-clawbot#1375's scope-stats fetch caching.

Fix (one line, public/app.js):

promise.finally(() => _inflight.delete(path)).catch(() => {});

The added .catch() only consumes the discarded .finally() promise's mirrored rejection. It is a different promise object from the one returned to callers, so:

• the original request's error still reaches the caller unchanged (proven by scenario B, matching the pre-fix error text exactly);
• caching, TTL, in-flight dedup, and 503 retry behavior are all unchanged (scenarios D, E, F, G).

Commits in this PR

1. d3c24830 — the production fix itself, plus the first version of the regression test.
2. 86645756 — hardens the test after an independent review found two test-harness false positives (an incomplete-suite exit-0 path, and scenario C accepting an unrelated child crash as a pass).
3. 98fbdbe2 — closes a further false positive in scenario H (a source-line echo containing the marker was mistaken for the real rejection) and fixes child-timeout labeling/SIGKILL handling, found in a second independent review round.
4. 34061595 — registers the now-approved test into test-all.sh and the CI JS-tests step in deploy.yml, immediately before issue bug(analytics): Scopes tab fails JSON.parse — '/api/api/scope-stats' duplicate prefix (#915 fix never merged) Kpa-clawbot/CoreScope#1375's existing (currently failing, unrelated) test.

Evidence

• Fail-before/pass-after: reproduced with node --unhandled-rejections=strict against a minimal harness loading the real, unmodified api() — before the fix, a caller correctly catches the rejection and the process still crashes afterward from the orphaned promise; after the fix, no crash.
• Regression test (test-app-api-inflight-cleanup-rejection.js) covers 8 scenarios (A–H): success, unchanged failure semantics, no extra unhandled rejection (via a self-checking child process under --unhandled-rejections=strict), in-flight cleanup after both success and failure, concurrent-request dedup, TTL cache behavior, and a self-check that the strict-mode detection technique itself actually detects a deliberate unhandled rejection (and doesn't just pass on any unrelated crash).
• Mutation testing (isolated scratch copies only, never the reviewed files): removing in-flight dedup, a typo'd api() reference, api() throwing before its fetch call, and reverting the production .catch() were each confirmed to produce a relevant, correctly-diagnosed test failure — not a vacuous pass or generic crash.
• Watchdog / child-timeout hardening: a non-unref'd watchdog guards against an async stall silently exiting 0; both spawned child processes use SIGKILL on timeout with a clear timeout diagnosis rather than a generic spawn-error message. (This is an event-loop-based safety bound for async stalls — it cannot interrupt spawnSync or other synchronous blocking in the main process; an outer runner-level timeout remains the relevant backstop for that case.)
• Independent review: the production fix and the final test harness (through 98fbdbe2) were both reviewed and approved in separate passes before the test was registered into CI.
• CI registration: the new test is registered in both test-all.sh and deploy.yml's JS-tests step, placed immediately before issue bug(analytics): Scopes tab fails JSON.parse — '/api/api/scope-stats' duplicate prefix (#915 fix never merged) Kpa-clawbot/CoreScope#1375's existing test so it runs and is visible regardless of that unrelated, pre-existing failure.

Known baseline / out of scope

• Issue bug(analytics): Scopes tab fails JSON.parse — '/api/api/scope-stats' duplicate prefix (#915 fix never merged) Kpa-clawbot/CoreScope#1375 (a separate, pre-existing stale-assertion test failure) is not fixed by this PR and is deliberately out of scope — it has its own separate branch/fix in progress.
• test-frontend-helpers.js has 2 pre-existing favStar assertion failures on unmodified master, unrelated to this change.
• test-live-dedup.js doesn't run in this environment (missing playwright dependency) — not a failure, just not run.
• No staging or production verification has been performed for this change.
• This fixes one specific orphaned-rejection source in api(). It does not claim that all unhandled rejections are eliminated, or that the rest of CI is green.

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