Python: fix(core): serialize Pydantic tool results in JSON mode - #9036

Motivation & Context

A tool that returns a Pydantic model is serialized with model_dump() and then json.dumps(default=str), so enum and datetime fields come out as their Python str():

class Ticket(BaseModel):
    status: Status        # Enum, Status.OPEN = "open"
    created: datetime

FunctionTool.parse_result(Ticket(status=Status.OPEN, created=datetime(2024, 1, 1, 12)))[0].text
# '{"status": "Status.OPEN", "created": "2024-01-01 12:00:00"}'
# Pydantic's JSON: {"status":"open","created":"2024-01-01T12:00:00"}

The model gets Status.OPEN back from a tool whose own input schema uses "open".

Description & Review Guide

• What are the major changes? _make_dumpable dumps BaseModel values in JSON mode: model_dump(mode="json", fallback=str) like _mcp.py, or on Pydantic < 2.11, which has no fallback, to_jsonable_python(model, by_alias=False, serialize_unknown=True), which uses the same JSON serializer. Either way Pydantic dumps the whole model, so serializers, computed fields, declared field types and exclude=True apply as usual and only values JSON mode doesn't know become str(). If Pydantic still can't dump the model (bytes that aren't valid UTF-8, a reference cycle), it is replaced by a placeholder and a warning is logged, instead of invoke falling back to str(result).
• What is the impact of these changes? Pydantic results (top level, in lists and dicts) are serialized the way Pydantic writes JSON. Plain str/int/float fields are unchanged.
• What do you want reviewers to focus on? FunctionTool._dump_model and the placeholder for models Pydantic can't dump.

Related Issue

N/A - trivial change: a bug fix in _make_dumpable (Pydantic tool results dumped in JSON mode). The repro is above and the PR adds a regression test that fails on main.

AI Assistance

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Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

Test: test_parse_result_pydantic_model_uses_json_mode in packages/core/tests/core/test_types.py; it fails on main. The parse_result tests pass on pydantic 2.0.3, 2.10.6 and 2.14.1 (the fallback tests run both dump paths). test_types.py, test_tools.py and test_function_invocation_logic.py pass locally except the two aiohttp tests (no aiohttp in my env). ruff check/format and pyright clean.