emrg: add session._validate_tool_messages tests

[argszero]

Add 10 tests for _validate_tool_messages — the safety-net function that post-processes tool call messages for OpenAI API validity.

Tests cover:

• Empty input
• Plain messages pass through
• Summary messages pass through
• Valid assistant + tool blocks preserved
• Orphaned tool_calls stripped (content preserved or set to "")
• Orphaned tool messages removed
• Partial match: only valid tool_calls kept
• Multiple assistant+tool blocks
• Mixed valid/orphaned blocks

[argszero]

✅ LGTM — cycle #29

[argszero]

✅ LGTM — cycle #30

[argszero]

✅ LGTM — cycle #31