fix(agent): preserve tool-call pairs during compaction

[brynary]

Summary

Fixes OpenAI Responses requests after context compaction by ensuring preserved tool results are not separated from the assistant tool calls that produced them. The previous fixed-size preserved tail could retain a function_call_output while dropping the matching function_call, which OpenAI rejects as an orphaned tool result.

Changes

• Extends History::compact so the preserved range moves backward until every kept tool result has its matching assistant tool call.
• Adds a unit invariant test for compacted histories that serialize tool results.
• Adds an OpenAI twin integration regression that forces compaction during a tool-use loop.
• Teaches the OpenAI twin to validate orphaned function_call_output items and script response usage counts for deterministic compaction tests.

Test Plan

• cargo +nightly-2026-04-14 fmt --check --all
• git diff --check
• FABRO_TEST_MODE=twin cargo nextest run -p fabro-agent --test it openai_twin_compaction_preserves_tool_call_pairs --run-ignored all
• cargo nextest run -p fabro-agent
• cargo nextest run -p twin-openai
• cargo nextest run -p fabro-test
• cargo +nightly-2026-04-14 clippy -p fabro-agent -p fabro-test -p twin-openai --all-targets --no-deps -- -D warnings

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