test: improve coverage with multi-parameter, partial parsing, and whitespace tests
- Add process_data_tool for testing multiple string parameters
- Add Seed OSS multi-parameter tool call test
- Add Seed OSS partial parsing test for incomplete string parameters
- Add MiniMax M2 compact format test (no whitespace)
- Add docs/test-divergence-analysis.md documenting test changes
These tests restore coverage that was reduced during PEG migration:
- Multi-parameter tests verify parsing of tools with multiple args
- Partial parsing tests verify streaming behavior
- Compact format tests verify whitespace flexibility
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>