test(mcpserver): cover PEP 604 union return annotations (#2591)

[Bartok9]

Summary

• Adds a regression test for tool functions whose return annotation is a multi-member PEP 604 union mixing container and scalar types (dict | list | str).
• Guards an already-fixed-but-untested case so it can't silently regress.

Motivation

Closes #2591.

FastMCP/MCPServer used to crash at registration with PydanticUserError when a tool's return type was a bare PEP 604 union like dict | list | str, because the types.UnionType was passed to create_model() as a field value.

This already works on current main (734746a): such a union is neither a types.GenericAlias nor a type, so _try_create_model_and_schema (func_metadata.py) falls through to the catch-all branch that calls _create_wrapped_model(...) with wrap_output=True; #2434 (5cbd259) additionally guards the schema-generation path. But there is no test covering the reported signature — the existing test_structured_output_generic_types only exercises a 2-member, all-scalar union (func_union() -> str | int), not a container+scalar mix with bare (unparametrized) generics, which is a different branch interaction (the dict member touches the RootModel dict special-casing before the union wraps).

The three prior fix attempts (#2592, #2599, #2669) were all closed during backlog cleanup rather than for an approach problem, and one of them switched the repro to typed generics to satisfy pyright — which masked the exact reported case. This PR keeps the bare-generic signature (with scoped # type: ignore) so the test reproduces the issue faithfully.

Verification

• uv run pytest tests/server/mcpserver/test_func_metadata.py -q — 34 passed
• uv run ruff check tests/server/mcpserver/test_func_metadata.py — clean
• uv run ruff format --check tests/server/mcpserver/test_func_metadata.py — already formatted
• uv run pyright tests/server/mcpserver/test_func_metadata.py — 0 errors
• Did NOT change: any source file. This is a test-only addition; the behavior it asserts already ships on main.

Real behavior proof

• Behavior addressed: @mcp.tool() async def my_tool(flag: bool) -> dict | list | str registering without PydanticUserError, and producing a wrapped anyOf output_schema.
• Real environment: Python 3.12, uv run, branch off upstream/main 734746a.
• Command run: uv run python -c "..." invoking func_metadata(func, structured_output=True).output_schema for () -> dict | list | str.
• Captured output (the schema the new test asserts):

  {
    "properties": {
      "result": {
        "anyOf": [
          {"additionalProperties": true, "type": "object"},
          {"items": {}, "type": "array"},
          {"type": "string"}
        ],
        "title": "Result"
      }
    },
    "required": ["result"],
    "title": "func_pep604_unionOutput",
    "type": "object"
  }

• Regression test: tests/server/mcpserver/test_func_metadata.py::test_structured_output_pep604_union_return — asserts the exact wrapped anyOf shape above for the issue's signature.
• What was NOT tested: runtime conversion of an actual returned value through this model (the existing structured-output round-trip tests already cover the wrap path generically); this test is scoped to the schema-generation path that previously raised.