fix: preserve Workers log source fields

[EfeDurmaz16]

Summary

• Preserve unknown object fields in Workers telemetry source payloads instead of stripping them during Zod parsing.
• Add a regression test for structured console.log() fields such as event, userId, and duration values.

Context

• Fixes Workers Observability: custom console.log fields not returned in events view #342.

Verification

• pnpm --filter @repo/mcp-common test -- workers-logs.types.spec.ts
• pnpm --filter @repo/mcp-common check:types
• pnpm prettier packages/mcp-common/src/types/workers-logs.types.ts packages/mcp-common/src/types/workers-logs.types.spec.ts --check
• git diff --check