fix(compression): exclude completion tokens from compression trigger (#12026)

Cherry-picked from PR #12481 by @Sanjays2402.

Reasoning models (GLM-5.1, QwQ, DeepSeek R1) inflate completion_tokens
with internal thinking tokens. The compression trigger summed
prompt_tokens + completion_tokens, causing premature compression at ~42%
actual context usage instead of the configured 50% threshold.

Now uses only prompt_tokens — completion tokens don't consume context
window space for the next API call.

- 3 new regression tests
- Added AUTHOR_MAP entry for @Sanjays2402

Closes #12026

tests/run_agent/test_compression_trigger_excludes_reasoning.py

@@ -0,0 +1,61 @@
+"""Verify compression trigger excludes reasoning/completion tokens (#12026).
+
+Thinking models (GLM-5.1, QwQ, DeepSeek R1) inflate completion_tokens with
+reasoning tokens that don't consume context window space.  The compression
+trigger must use only prompt_tokens so sessions aren't prematurely split.
+"""
+
+import types
+import pytest
+from unittest.mock import MagicMock, patch
+
+
+def _make_agent_stub(prompt_tokens, completion_tokens, threshold_tokens):
+    """Create a minimal stub that exercises the compression check path."""
+    compressor = types.SimpleNamespace(
+        last_prompt_tokens=prompt_tokens,
+        last_completion_tokens=completion_tokens,
+        threshold_tokens=threshold_tokens,
+    )
+    # Replicate the fixed logic from run_agent.py ~line 11273
+    if compressor.last_prompt_tokens > 0:
+        real_tokens = compressor.last_prompt_tokens  # Fixed: no completion
+    else:
+        real_tokens = 0
+    return real_tokens, compressor
+
+
+class TestCompressionTriggerExcludesReasoning:
+    def test_high_reasoning_tokens_should_not_trigger_compression(self):
+        """With the old bug, 40k prompt + 80k reasoning = 120k > 100k threshold.
+        After the fix, only 40k prompt is compared — no compression."""
+        real_tokens, comp = _make_agent_stub(
+            prompt_tokens=40_000,
+            completion_tokens=80_000,  # reasoning-heavy model
+            threshold_tokens=100_000,
+        )
+        assert real_tokens == 40_000
+        assert real_tokens < comp.threshold_tokens, (
+            "Should NOT trigger compression — only prompt tokens matter"
+        )
+
+    def test_high_prompt_tokens_should_trigger_compression(self):
+        """When prompt tokens genuinely exceed the threshold, compress."""
+        real_tokens, comp = _make_agent_stub(
+            prompt_tokens=110_000,
+            completion_tokens=5_000,
+            threshold_tokens=100_000,
+        )
+        assert real_tokens == 110_000
+        assert real_tokens >= comp.threshold_tokens, (
+            "Should trigger compression — prompt tokens exceed threshold"
+        )
+
+    def test_zero_prompt_tokens_falls_back(self):
+        """When provider returns 0 prompt tokens, real_tokens is 0 (fallback path)."""
+        real_tokens, _ = _make_agent_stub(
+            prompt_tokens=0,
+            completion_tokens=50_000,
+            threshold_tokens=100_000,
+        )
+        assert real_tokens == 0