fix(python): serialize native values in tool results

[OllieinCanada]

Fixes #2203

Summary

Complete the plain-container serialization case intentionally left out of #2225. Python tool handlers can now return dictionaries containing common native values without those successful results being converted into opaque tool failures.

Fix

Extend the existing json.dumps fallback with the same JSON-safe representations used for Pydantic tool results:

• datetime, date, and time values use ISO 8601 strings
• UUID and Decimal values use strings
• Enum values use their underlying value
• sets use JSON arrays

Unsupported objects continue to raise the existing clear serialization error.

Tests

The regression test failed on current main with:

TypeError: Failed to serialize tool result: Object of type UUID is not JSON serializable

Passing validation:

• Python 3.13: 42 tool tests passed
• Python 3.11 (the oldest CI version): 42 tool tests passed
• complete offline root suite: 209 tests passed
• Ruff formatting check passed
• Ruff lint passed
• ty check copilot passed
• git diff --check passed

No Copilot session, model, paid API, or external service is required for the regression test. Authenticated runtime E2E tests were not run locally.