## Summary
This PR fixes a deterministic mismatch in remote compaction where
pre-trim estimation and the `/v1/responses/compact` payload could use
different base instructions.
Before this change:
- pre-trim estimation used model-derived instructions
(`model_info.get_model_instructions(...)`)
- compact payload used session base instructions
(`sess.get_base_instructions()`)
After this change:
- remote pre-trim estimation and compact payload both use the same
`BaseInstructions` instance from session state.
## Changes
- Added a shared estimator entry point in `ContextManager`:
- `estimate_token_count_with_base_instructions(&self, base_instructions:
&BaseInstructions) -> Option<i64>`
- Kept `estimate_token_count(&TurnContext)` as a thin wrapper that
resolves model/personality instructions and delegates to the new helper.
- Updated remote compaction flow to fetch base instructions once and
reuse it for both:
- trim preflight estimation
- compact request payload construction
- Added regression coverage for parity and behavior:
- unit test verifying explicit-base estimator behavior
- integration test proving remote compaction uses session override
instructions and trims accordingly
## Why this matters
This removes a deterministic divergence source where pre-trim could
think the request fits while the actual compact request exceeded context
because its instructions were longer/different.
## Scope
In scope:
- estimator/payload base-instructions parity in remote compaction
Out of scope:
- retry-on-`context_length_exceeded`
- compaction threshold/headroom policy changes
- broader trimming policy changes
## Codex author:
`codex fork 019c2b24-c2df-7b31-a482-fb8cf7a28559`
Charley Cunningham
dc7007beaa