Adds comprehensive acceptance tests for plan §10 OpenTelemetry tracing:
- Verify tracing.enabled=false returns None (zero overhead)
- Verify default config has tracing disabled
- Verify sample_rate config parsing (default 10%)
- Verify resource attributes (service.name, endpoint, POD_NAME)
- Verify feature flag controls compilation
- Verify shutdown_otel is safe to call multiple times
- Verify span hierarchy exists in scatter path code
- Verify TracingConfig serde round-trip (JSON/TOML)
Also makes the otel module public via lib.rs for test access,
and adds toml as a dev dependency for config parsing tests.
All 15 tests pass. Closes: miroir-afh.6
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
0b266bf37e