fix: omit empty task result fields

[he-yufeng]

Summary

• serialize completed task results with exclude_none=True and JSON mode before adding related-task metadata
• keep empty optional content fields out of the task result payload, matching normal session response serialization
• add a regression assertion for TextContent task results so annotations and _meta are omitted instead of serialized as null

Fixes #2539.

To verify

• python -m py_compile src/mcp/server/experimental/task_result_handler.py tests/experimental/tasks/server/test_task_result_handler.py
• uv run python -m pytest tests/experimental/tasks/server/test_task_result_handler.py -q
• uv run ruff check src/mcp/server/experimental/task_result_handler.py tests/experimental/tasks/server/test_task_result_handler.py
• git diff --check