fix(compression): dedup duplicate tool_call_ids to prevent HTTP 400 on strict providers

[isheng-eqi]

What does this PR do?

Fixes a context compression bug where duplicate tool_call_id values survive into the post-compression message list, causing strict providers (DeepSeek) to reject the API call with HTTP 400 and abort the session.

Related Issue

Fixes #58327

Type of Change

• Bug fix

Changes Made

• agent/context_compressor.py: Extend _sanitize_tool_pairs() with a dedup pass (step 1b) that removes duplicate tool results sharing the same tool_call_id. Walks the message list in reverse so the most recent result (closest to the tail) is always preserved.
• tests/agent/test_context_compressor_tool_dedup.py: 6 tests covering single duplicates, multiple duplicates, no-op, dedup+orphan interaction, and non-tool message preservation

How to Test

1. Run a long session with many tool calls on DeepSeek
2. Trigger context compression
3. Verify the session continues normally instead of aborting with HTTP 400 'Duplicate value for tool_call_id'

[isheng-eqi]

Branch contaminated — rebuilding with clean base from upstream/main.