Add event_tags and event_properties callbacks

[kashishhora]

What

Two new callbacks on MCPCatOptions — event_tags and event_properties — that let customers attach custom metadata to every auto-captured event (initialize, tools/list, tools/call). Ports the equivalent feature from the TypeScript SDK (PR #32 there).

• event_tags: dict[str, str], validated client-side (regex on keys, length caps, max 50 entries, no newlines in values). Invalid entries are dropped with a warning.
• event_properties: dict[str, Any], arbitrary JSON-serializable metadata, no validation.
• Both bypass redaction/sanitization/truncation — customer-supplied, passed through as-is.
• Both accept sync or async callables.

Why

Customers want to filter/group events by structured metadata they control — trace IDs, environments, deployment regions, feature flags. Until now there's no way to attach that kind of data through the SDK.

Exporter changes worth flagging

• Every outbound event now carries source=mcpcat (new MCPCAT_SOURCE constant). Makes MCPcat events distinguishable from other telemetry flowing into the customer's observability backend.
• Customer tags are namespaced — mcpcat.<key> in Datadog ddtags and Sentry tags, mcpcat.tag.<key> as OTLP span attributes. Prevents collisions with reserved tag keys (environment, release, host, service, etc.).
• Sentry properties moved from the deprecated extra field into contexts.mcpcat.

Test plan

• uv run python -m pytest tests/test_event_tags_properties.py tests/test_tag_validation.py -v — 34 tests pass
• uv run python -m pytest — full regression green
• Manually verified end-to-end against local BE: SDK sends events with tags/properties → SQS → process_events → event detail API returns them. Confirmed for both sync and async callback paths. Covered in the separate BE change that exposes tags/properties on EventSerializer.

Notes

• Async callback support uses the same sync-or-async pattern as RedactionFunction. If the callback returns a coroutine, the resolver awaits it; otherwise passes through.
• Validation rules match the TS SDK and the backend's validate_tags exactly, so client- and server-side drop behavior is consistent.
• identify remains sync-only for now — making it async-compatible is a separate (small) change.