Add Python customized tools guide, example, and SDK tool re-exports

[RealKai42]

• Add guides/python/customized-tools.md with a full custom tool walkthrough and SDK-based imports.
• Add a runnable examples/python/customized-tools sample with agent file, tool module, and entrypoint.
• Re-export CallableTool2, ToolOk, ToolError, ToolReturnValue from the Python SDK and update docs/examples to use them.
• Add python/tests/test_customized_tools.py to verify agent loading of a custom tool.
• Link the agent file docs from guides/python/prompt.md and guides/python/session.md.

[Copilot]

Pull request overview

This pull request adds comprehensive support for custom tools in the Python SDK by re-exporting tool-related classes from the underlying kosong.tooling module and providing complete documentation and examples. The PR enables SDK users to define custom tools using the same approach as Kimi CLI.

Changes:

• Re-exported CallableTool2, ToolOk, ToolError, and ToolReturnValue from kosong.tooling in the SDK's public API
• Added a complete guide at guides/python/customized-tools.md with step-by-step instructions for creating custom tools
• Created a runnable example in examples/python/customized-tools/ demonstrating custom tool implementation
• Added tests to verify the tool exports and agent file loading functionality
• Updated prompt.md and session.md to link to the Custom Agent Files documentation

Reviewed changes

Copilot reviewed 11 out of 12 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
python/src/kimi_agent_sdk/init.py	Re-exports CallableTool2, ToolOk, ToolError from kosong.tooling and moves ToolReturnValue import source
python/tests/test_customized_tools.py	Tests verifying tool exports match kosong and custom tool loading via agent files
guides/python/customized-tools.md	Comprehensive guide for implementing and registering custom tools
guides/python/prompt.md	Added link to Custom Agent Files documentation for agent_file parameter
guides/python/session.md	Added link to Custom Agent Files documentation for agent_file parameter
guides/python/quickstart.md	Added "What's Next" section linking to guides including customized-tools
examples/python/customized-tools/*	Complete runnable example with custom Ls tool, agent file, and entrypoint

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