fix: truncate oversized tool_call IDs for cross-provider compatibility

[themavik]

Summary

Fixes #2215

When using multiple model backends (e.g. vLLM + OpenAI) within the same agent, tool_call IDs generated by one backend can exceed the maximum length accepted by another. OpenAI rejects IDs longer than 40 characters with:

Invalid 'messages[2].tool_calls[0].id': string too long

Root Cause

vLLM and other backends may generate tool_call IDs longer than 40 characters (e.g. UUIDs). When these are stored in chat history and later replayed against OpenAI, the request is rejected.

Changes

Added _normalize_tool_call_ids() to BaseModelBackend that truncates oversized tool_call IDs in preprocess_messages(). This ensures:

• Assistant message tool_calls[].id fields are truncated to 40 chars
• Tool response tool_call_id fields are truncated consistently
• Truncation is deterministic (same input always gives same prefix)
• Only activates when IDs exceed the limit — no-op for conforming IDs

The normalization runs in preprocess_messages() which all model backends use, so cross-provider compatibility is handled transparently.

Test Plan

• Verify OpenAI model accepts messages with previously-vLLM-generated tool_call IDs
• Verify truncation is consistent between assistant and tool response messages
• Verify no regression when all IDs are already under 40 characters

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