fix: harden Sentry/PostHog overload-error filters (closes TONALCOACH-1C)

[JeffOtano]

Summary

Reviewed open Sentry issues and found TONALCOACH-1C — 807 occurrences of "This model is currently experiencing high demand" still reaching Sentry despite sentryBeforeSend having a suppression entry for that substring. Three root-cause gaps identified and fixed:

1. sentryBeforeSend.ts — cause chain not inspected (src/lib/sentryBeforeSend.ts)

The Vercel AI SDK wraps provider errors: the outer Error carries a generic message (e.g. "Error reading UI message stream") while the Gemini-specific text lives on Error.cause. The errorMessages helper only checked the outer .message, so the suppression filter matched the generic wrapper and let the event through.

Fix: Walk the full Error.cause chain and add every ancestor's message to the candidate set.

2. sentryBeforeSend.ts — non-Error objects not handled

SDK error types that implement the Error interface structurally (without extends Error) weren't inspected, so hint.originalException instanceof Error returned false and the message was silently skipped.

Fix: Duck-type the check — if originalException is a plain object with a string .message, include that message.

3. Missing Gemini 503 variant: "model is currently overloaded"

Gemini has a second 503 overload phrasing ("The model is currently overloaded. Please try again later.") that maps to provider_overload on the backend but had no suppression entry in either sentryBeforeSend or posthogBeforeSend.

Fix: Add "model is currently overloaded" to both filter lists.

4. finalizePendingMessages now idempotent (convex/ai/resilience.ts)

When @convex-dev/agent's internal stream-error handler finalizes a message before our retry loop reaches it, the subsequent finalizeMessage call throws, aborting the loop and leaving other pending messages unprocessed.

Fix: Wrap each per-message finalizeMessage call in try/catch — the catch is intentionally silent since this is a best-effort cleanup pass on an already-failed turn. The outer safeFinalizePending still catches any remaining errors.

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Test plan

• All new Sentry suppression paths covered: cause-chain one level deep, two levels deep, non-Error object, new "model is currently overloaded" pattern, kept-real-error guard
• posthogBeforeSend gains matching "model is currently overloaded" test
• resilienceFinalize.test.ts adds: new overloaded → provider_overload mapping, exhaustive sanitized-code table for all transient kinds
• CI passes (typecheck, lint, vitest)
• Close TONALCOACH-1C in Sentry after merge

https://claude.ai/code/session_018xtWrKbmTQswzitpVKoLij

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