fix(backend): fail fast on disconnected sessions + honest error taxonomy

[gcharang]

Summary

Closes the three pre-existing QA follow-ups from the attended M3 e2e, plus the hardening the review lanes drove on top. Pure backend change; protocol/extension/connector untouched.

1. No-extension fail-fast (503, was a 30s-timeout 500)

CfSessionCoordinator.send() now rejects immediately unless the host reports a live, onConnect-authorized extension socket (CoordinatorHost.hasAuthorizedConnection()), and the commands route maps that to 503 {error:"extension not connected"}.

Two load-bearing details:

• The gate reads the delivery predicate, not SessionState.status. The status scalar is a lossy last-writer-wins echo: a late onClose from a replaced socket stamps detached over a healthy reconnected session, which would wedge it into permanent 503 with no backstop (architect-lane catch). onClose now also only detaches when no other authorized socket remains.
• Non-2xx on purpose. A 200 ok:false Event would be cached by consumers' breakwater idempotency stores and replayed even after the extension reconnects; a 503 makes @understudy/connector's callUnderstudy throw, releasing the reservation so the retry re-executes.

Ref-less dryRun writes (e.g. navigate) keep their documented zero-wire simulated-ok semantic — regression-tested.

2. Honest error taxonomy at the route

Mapped by message-prefix constants in the portable src/coordinator.ts (a prefix is the only failure signal that survives the DO RPC boundary): 503 not-connected · 504 {error:"command timed out"} · 400 {error:"invalid body"} for unparseable JSON (was a masked 500) · uniform JSON 500 via app.onError for everything else.

3. The rest

• fill_secret checks the predicate before resolveSecret — no vault read / plaintext materialization for an undeliverable command (DL-004), spy-tested.
• Dry-run probe failures carry the extension's own reason (dry-run: stale or unknown ref: s1e9, unexpected probe response '<type>') instead of one collapsed string.
• Env.CALLER_TOKENS/EXTENSION_TOKENS are now required types (they were always in secrets.required; wrangler deploy --dry-run validates clean). Runtime empty-string guards stay.

Test plan

• Backend 53 → 64 tests (5 files): fail-fast unit test; full lifecycle pending→connected→detached→reconnected; replaced-socket close ≠ detach; vault-untouched disconnected fill_secret; probe-reason fidelity incl. wrong-event-type branch; 400/500 route tests; the old 35s DO-timeout test converted into the route-level 504 proof (same wall-clock cost, full route→RPC→DO path, awaiting-marker clear re-asserted)
• Full workspace: pnpm -r typecheck (4/4) + pnpm -r test = 142 passed
• Quality gate: quality-reviewer APPROVE, architect should-fixes fixed with regression tests, QA round-2 adversarial SIGN-OFF (3-socket close orders, bad-token sockets never authorized, prefix-collision survey of every throw site)

Note for wrangler tail readers: fail-fast rejections print Uncaught (in promise) via workerd's DO RPC instrumentation — these are handled 503s, not real uncaught errors.

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