feat!(ContentToolResult): add a new model_format parameter

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[Copilot]

Pull Request Overview

This PR adds a new model_format parameter to ContentToolResult to allow specifying how tool results should be formatted for the chat model, and it migrates JSON handling from the standard json library to orjson across the codebase.

• Adds a new test (test_anthropic_image_tool) to validate tool result handling for images.
• Updates ContentToolResult (and related functions) to support configurable formatting via model_format.
• Replaces usage of json with orjson in several modules and adds orjson as a dependency.

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/test_provider_anthropic.py	Adds tests for a new image tool using ContentToolResult with the model_format parameter.
tests/conftest.py	Introduces a fixture for test image directory references.
pyproject.toml	Includes orjson as a dependency.
chatlas/_openai.py	Migrates JSON serialization to orjson; serializes arguments in function messages.
chatlas/_ollama.py	Updates JSON parsing to use orjson for model tag retrieval.
chatlas/_google.py	Converts JSON handling for content to use orjson instead of json.
chatlas/_content.py	Introduces custom formatting methods (_get_display_value and get_model_value) for tool results.
chatlas/_auto.py	Updates environment variable parsing to use orjson.
chatlas/_anthropic.py	Adjusts JSON parsing for streamed messages to use orjson and updates tool result formatting.
CHANGELOG.md	Documents the new model_format parameter feature and the migration to orjson.