feat: add Claude 4.6 models and fix OpenClaw 422 validation errors

[bhargav18]

Summary

• Add claude-sonnet-4-6 and claude-opus-4-6 to CLAUDE_MODELS and set claude-sonnet-4-6 as the new DEFAULT_MODEL
• Fix 422 validation errors when using OpenClaw as a client by accepting role: 'tool' messages, null content, and arbitrary content block types
• Add model_config extra='ignore' to silently drop unknown fields (e.g. store, service_tier) sent by OpenClaw

Changes

src/constants.py

• Added Claude 4.6 models (claude-sonnet-4-6, claude-opus-4-6) at the top of CLAUDE_MODELS
• Updated DEFAULT_MODEL from claude-sonnet-4-5-20250929 to claude-sonnet-4-6

src/models.py

• Message.role: added "tool" to allowed roles — OpenClaw sends tool result messages with role: "tool"
• Message.content: changed to Optional[Union[str, List[Any]]] to handle null content (assistant messages mid-tool-call) and arbitrary content block arrays
• ContentPart: loosened type from Literal["text"] to str and made text optional to handle non-text block types (tool_use, tool_result, image, etc.)
• normalize_content: added null handling and text extraction from tool_result content blocks
• Added model_config = {"extra": "ignore"} to both ContentPart and Message to silently drop unknown fields

Test plan

• All existing tests pass: poetry run python tests/test_endpoints.py → 4/4
• All existing tests pass: poetry run python tests/test_basic.py → 4/4
• Server starts cleanly with poetry run uvicorn src.main:app --reload --port 8000
• Verify OpenClaw requests no longer return 422 with a live OpenClaw instance

🤖 Generated with Claude Code