Add agent chat action support

[ks93]

Description

This PR introduces support for actions within the client.agents.chat feature. This enables more dynamic interactions with AI agents by allowing clients to:

• Provide client-side tools (actions) to the agent: The chat method now accepts an actions parameter, allowing users to define ClientToolAgentActionWrite objects that the agent can request to execute.
• Receive action requests from the agent: The AgentMessage in the chat response can now include a list of actions (e.g., ClientToolResponseAction, ToolConfirmationResponseAction), indicating that the agent wants the client to perform a specific operation.
• Send action results back to the agent: The Message dataclass has been extended to support an "action" role and action_id, allowing users to send the results of a client-executed action back to the agent for continued conversation.

This implementation provides a type-safe and extensible abstraction for handling agent actions, aligning with existing SDK patterns for tools. Comprehensive examples are included in the chat method's docstring to guide users.

Checklist:

• Tests added/updated.
• Documentation updated. Documentation is generated from docstrings - these must be updated according to your change.
  If a new method has been added it should be referenced in cognite.rst in order to generate docs based on its docstring.
• Changelog updated in CHANGELOG.md.
• Version bumped. If triggering a new release is desired, bump the version number in _version.py and pyproject.toml per semantic versioning.

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