Preserve the primary hook failure during shutdown

[wallacelim]

Problem

With fail_on_error=true, a rollout-conversion failure is thrown from the hook's try block. If telemetry shutdown then also fails in finally, the shutdown error replaces the original conversion error, hiding the root cause.

Change

Capture both failures while still always attempting shutdown, then rethrow the conversion failure first. A shutdown-only failure continues to propagate, and the default fail-open behavior is unchanged.

This is intentionally separate from usage accounting (#28), hook path portability (#33), and service-tier pricing (#34).

Validation

• corepack pnpm@9.5.0 test — 36 passing
• corepack pnpm@9.5.0 run lint — Prettier, TypeScript, and generated-bundle checks pass

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