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
