fix(core): serialize Pydantic tool results in JSON mode

[Hulk (hulkbig)]

Why are these changes needed?

A tool can complete successfully but still be reported as failed when it returns a Pydantic model containing a datetime, UUID, or Decimal. BaseTool.run_json() formats the result while constructing its ToolCallEvent. The current model_dump() call keeps these values as Python objects, so the following json.dumps() raises TypeError after the tool has already executed. StaticWorkbench then returns that serialization exception with is_error=True.

Use model_dump(mode="json") so Pydantic converts supported field types and applies JSON-only serializers before encoding the result. Keep the existing dictionary/string handling so a custom model serializer that returns a string remains unquoted.

Regression coverage includes top-level and nested datetime/UUID/Decimal values, a JSON-only field serializer, and a successful StaticWorkbench call. The execution-path tests assert that the tool executes exactly once. The existing custom string-returning model serializer test remains unchanged and passes.

Related issue number

No linked issue. This fixes the Pydantic model branch; it does not change plain dict/list output handling discussed in #7867 and the related open PRs.

Validation

• Added regression cases: 8 failed before the production change; all pass afterward
• autogen-core package suite: 234 passed on Python 3.12.14 and the lockfile's Pydantic 2.10.6
• Package-wide poe lint, ruff format --check src tests, poe pyright, and poe mypy passed
• git diff --check passed

Tests were run through uv with the repository's pinned check-tool versions. The package test command was OTEL_TRACES_SAMPLER=always_on PYTEST_XDIST_AUTO_NUM_WORKERS=4 uv run --no-sync poe test. Explicit always-on sampling overrides the test environment's 1% sampling configuration so the existing tracing assertions are deterministic. No model-service credentials or live LLM calls were used. The full monorepo aggregate checks and documentation builds were not run.

Checks

• No documentation change needed for this internal serialization bug fix
• Added regression tests corresponding to the change
• All remote checks have passed (not run yet)

AI assistance

AI assistance was used to develop the fix and regression tests and draft this description. The local validation results and limitations are listed above.

[Hulk (hulkbig)]

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