[hylin (linhongyu510)]

Title

fix(python): preserve non-adjacent tool pairs during history truncation

Motivation and Context

• Problem: ChatHistoryTruncationReducer could retain a FunctionResultContent while dropping its matching FunctionCallContent when the two were separated by non-function messages.
• Existing contract: adjacent tool blocks were already protected, and matched ID pairs are used elsewhere by the reducer utilities. A retained tool result should not become an orphan after truncation.
• Why this is minimal: the fix chooses the existing user-threshold candidate boundary first, then applies a final matched-ID pair closure.
• Scope: offline chat-history reduction only; no provider, network, E2E, public API, dependency, lockfile, or documentation-only refactor.
• Issue: self-discovered during audit; independent dedup found no equivalent open issue/PR.

Description

locate_safe_reduction_index now builds an O(n) suffix minimum-call map from get_call_result_pairs(history). After the existing adjacency and threshold-window candidate selection, it moves the cut back only when a retained result requires an earlier matched call. Chained non-adjacent pairs are closed transitively. If the required call is before offset_count, reduction returns None rather than crossing offset/summary territory.

Offset, threshold, system/developer preservation, unknown/unpaired content, and existing user-boundary search semantics are otherwise unchanged.

Local verification

• Base: 483ffb487a33a1644886992d56599feae3b69b76.
• Pre-fix formal regression: 4 failed, 4 passed.
• Post-fix formal regression: 8 passed.
• Production-only revert cycle: red exit 1, restore green exit 0.
• Related chat-history suite: 105 passed.
• Ruff: 0.9.10; check and format check pass.
• Mypy: targeted changed production file passes.
• Multi-Python matrix and wheel build were not run in this preparation step.
• AI assistance was used; an independent local review was rerun before publication. No external CLI review is claimed.

Contribution Checklist

• The code builds clean without any errors or warnings in the focused local checks.
• The PR follows the Semantic Kernel Contribution Guide and local Ruff/Mypy checks.
• All unit tests pass, and I added focused tests for the non-adjacent call/result regression.
• I did not break anyone.
• AI assistance is disclosed.