Fix task result payload null fields

[he-yufeng]

Summary

• serialize stored task results with mode="json", exclude_none=True
• keep tasks/result payloads from emitting optional fields as explicit null
• add a regression test for TextContent results returned through TaskResultHandler

Fixes #2539.

To verify

• uv run pytest tests\experimental\tasks\server\test_task_result_handler.py::test_handle_omits_none_fields_from_result_payload tests\experimental\tasks\server\test_task_result_handler.py::test_handle_returns_result_for_completed_task -q --basetemp .tmp\pytest -p no:cacheprovider
• uv run ruff check src\mcp\server\experimental\task_result_handler.py tests\experimental\tasks\server\test_task_result_handler.py
• uv run ruff format --check src\mcp\server\experimental\task_result_handler.py tests\experimental\tasks\server\test_task_result_handler.py
• uv run pyright src\mcp\server\experimental\task_result_handler.py tests\experimental\tasks\server\test_task_result_handler.py
• uv run python -m py_compile src\mcp\server\experimental\task_result_handler.py tests\experimental\tasks\server\test_task_result_handler.py
• git diff --check