feat: add Notion tool README, improve tool logic, and expand test coverage

[Antiarin]

Description

Add documentation, new tools, and comprehensive test coverage for the Notion integration.

Type of Change

• New feature (non-breaking change that adds functionality)
• Documentation update

Related Issues

Fixes #5872

Changes Made

• Add README.md with setup instructions, tool reference table, usage examples for all 8 tools, and error handling guide
• Add notion_update_page tool for updating page properties and archiving/unarchiving
• Add notion_get_block_children tool for reading page body content (blocks)
• Add notion_append_blocks tool for appending paragraph blocks to a page
• Add title_property parameter to notion_create_page to support databases with custom title columns
• Register new tools in the credential adapter
• Expand test suite from ~60 lines to ~550 lines covering all 8 tools, error handling, credential store, edge cases, and input validation

Testing

• Unit tests pass (cd tools && uv run pytest tests/tools/test_notion_tool.py)
• All new tools have dedicated test classes with positive and negative cases

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes