fix(events): never drop events on non-JSON-serializable payloads

[naji247]

Summary

• Bug (1.0.1): events whose payload contained a value that is not JSON-serializable were being silently dropped. Two failure modes, one root cause:
  • Truncation aborted with Unable to serialize unknown type: <class 'function'> — truncate_event serializes the event with pydantic's model_dump_json(), which raises on a raw callable.
  • The generated API client then failed to send the event ('set' object has no attribute '__dict__') and, after three identical retries, dropped it.
• Both originated from the FastMCP v3 middleware capturing a tools/list response via Tool.model_dump(), which includes SDK-internal fields that are not JSON-serializable: a callable fn (FunctionTool) and a tags set.
• Fix (two layers):
  • Source — _tool_to_dict now emits the canonical MCP shape via to_mcp_tool() (name/description/inputSchema/…), so the tools/list payload is clean.
  • Systemic guarantee — truncate_event coerces event payload fields to JSON-safe primitives up front (callables → repr, sets → lists, datetimes → ISO strings, unknown → str) via a new make_json_safe helper, and passes fallback=str to its size-check serialization. Because the API client re-serializes the same event object on send, this prevents any non-serializable value in a payload from dropping an event.
• Release: patch version bump 1.0.1 → 1.0.2.

Impact

Customers on 1.0.1 running FastMCP v3 servers were losing events at scale — tools/list telemetry in particular was serialized, failed to send, and dropped after retries. This restores delivery for those events and hardens the pipeline against the whole class of non-serializable payloads (tool results containing datetimes, sets, or custom objects; custom event properties).

Test plan

• uv run pytest tests/test_truncation_json_safe.py tests/community/test_community_v3_event_serialization.py -v — new unit + end-to-end tests pass:
  • truncation does not abort on a callable/set payload and yields a JSON-serializable event
  • make_json_safe neutralizes callables, sets, and datetimes
  • a real tools/list event is JSON-serializable, is the clean MCP shape (no fn), and survives the generated API client's serializer (the exact 'set' failure is gone)
• Regression check: on unfixed code all these tests fail — the end-to-end one fails with the exact production error 'set' object has no attribute '__dict__'; disabling either fix layer independently re-breaks a test
• uv run pytest tests/ -q — full suite: 508 passed, 4 skipped, 3 xfailed
• Verified on FastMCP 3.0.0b1 (pinned) and 3.4.4 (latest published)