test(shared): direct coverage for the blockchain event bus and HTTP metrics hook

[hartz0]

Summary

Two pieces of shared infrastructure that every module depends on had no dedicated test — coverage was entirely incidental, coming from module-level tests that call handlers directly or from a full HTTP integration test that happens to run the plugin as a side effect. This PR adds a spec file for each, testing the component's own contract at its own boundary.

src/shared/events/index.spec.ts — closes #125

The in-process publishBlockchainEvent / onBlockchainEvent bus that all seven event-consuming modules subscribe to. New tests, all going through the real bus rather than a handler:

• a published event reaches every current subscriber (fan-out)
• calling the unsubscribe function returned by onBlockchainEvent stops further delivery to that listener only, leaving others attached
• publishing with zero subscribers is a no-op, not an error
• multiple publishBlockchainEvent calls are each delivered independently — not batched or coalesced
• calling an unsubscribe function twice is harmless

The unsubscribe test is a genuine regression guard: if onBlockchainEvent's teardown stopped detaching the listener, the "without affecting others" assertion (leaves called exactly once) fails.

src/shared/http/plugins/metrics.spec.ts — closes #126

The onResponse hook that records http_requests_total / http_request_duration_seconds. New tests build a minimal Fastify instance with one parameterized route and assert:

• a request to a concrete URL (/api/v1/deliveries/abc-123) is recorded under the route pattern (/api/v1/deliveries/:chainDeliveryId), and the concrete ids never become their own time series — the label-cardinality guard the file's header comment describes
• a request to an unregistered path (404) is recorded with route="unmatched" rather than dropped

Assertions read the shared prom-client registry via a before/after delta, since it accumulates across the suite.

Testing

• pnpm exec vitest run src/shared/events/index.spec.ts src/shared/http/plugins/metrics.spec.ts → 7 passing
• pnpm exec eslint clean on both new files
• tsc --noEmit introduces no new errors

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