Remove timeline metadata diagnostics (#24660)

## Summary

- remove the timeline-specific metadata metric and its rate-limiting
diagnostics service
- drop the MetricsModule dependency from TimelineActivityModule
- delete diagnostic collection/reporting plumbing from routing plans,
type resolution, and activity execution
- keep the existing fail-closed behavior: invalid or ambiguous contracts
still do not route or write activities

This removes 194 net lines and avoids carrying a metric and in-memory
report cache for a rare configuration condition.

## Stack

Base: `codex/timeline-coalescing-lock` (#24659)

## Validation

- `npx jest --config ./jest.config.mjs --runInBand src/modules/timeline`
(from `packages/twenty-server`) — passed, 15 suites / 79 tests
- `npx tsgo -p packages/twenty-server/tsconfig.json --noEmit` — passed
- `npx oxlint --type-aware -c packages/twenty-server/.oxlintrc.json
packages/twenty-server/src/engine/core-modules/metrics/types/metrics-keys.type.ts
packages/twenty-server/src/modules/timeline/services
packages/twenty-server/src/modules/timeline/timeline-activity.module.ts`
— passed
- `npx oxfmt --check
packages/twenty-server/src/engine/core-modules/metrics/types/metrics-keys.type.ts
packages/twenty-server/src/modules/timeline/services
packages/twenty-server/src/modules/timeline/timeline-activity.module.ts`
— passed
- `git diff --check` — passed


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