fix: accept system role in messages[] and handle built-in Anthropic tools

[HtetOoWaiYan]

Summary

Two fixes for 422 validation errors introduced by newer Claude models and Anthropic built-in tools.

Fix 1: Accept system role in messages[] (closes #26)

ClaudeMessage.role was Literal["user", "assistant"], which caused Pydantic to reject requests with a 422 before they reached the converter.

Newer Claude models (e.g. claude-opus-4-8) send interleaved system-role messages inside the messages[] array, not only via the top-level system field. Example request that was being rejected:

{
  "messages": [
    {"role": "user",   "content": "..."},
    {"role": "system", "content": "..."},
    {"role": "user",   "content": "..."}
  ]
}

Changes:

• ClaudeMessage.role now accepts "system" as a valid value
• request_converter.py handles role: "system" messages in the loop, converting them to OpenAI system messages instead of silently dropping them

Fix 2: Handle built-in Anthropic tools (closes #67)

Built-in Anthropic tools like web_search_20250305 and computer_use_20251201 are sent without an input_schema. Since ClaudeTool.input_schema was a required field, this caused a 422 immediately.

Changes:

• ClaudeTool.input_schema is now Optional (defaults to None)
• New optional fields type and max_uses added to ClaudeTool to accept the full built-in tool shape
• The converter skips tools with no input_schema (logging a debug message) rather than attempting to forward them to the OpenAI endpoint, which wouldn't understand them anyway

Note: Full execution of built-in tools (web search, computer use) would require the proxy to act as a tool executor, which is out of scope for this fix. This change prevents the crash and degrades gracefully.

Test plan

• Send a request with a system-role message inside messages[] — should return 200 instead of 422
• Send a request with tools: [{"type": "web_search_20250305", "name": "web_search", "max_uses": 3}] — should return 200 instead of 422 (tool will be skipped/not forwarded)
• Normal user/assistant/tool-use flows unaffected