fix: keep tool call message pairs when trimming context

[CompilError-bts]

Motivation

• Prevent context-trimming from leaving isolated tool messages at the front of context which can cause invalid requests during retry after context-length errors (issue [Bug]上下文管理截断时未保持 tool_calls 与 tool 消息的配对完整性，导致 400 报错 AstrBotDevs/AstrBot#7225).

Description

• Replace the previous naive two-message pop logic in AbstractProvider.pop_record with atomic-unit removal that treats an assistant message containing tool_calls plus its following tool messages as one unit.
• Handle leading orphan tool messages by removing contiguous leading tool messages as a unit.
• Implement helper functions (_has_tool_calls, _first_non_system_index, _pop_earliest_unit) and loop until at least two non-system records worth of content have been removed or no more removable units exist.
• Add two unit tests in tests/test_openai_source.py to cover atomic removal of assistant(tool_calls)+tool pairs and cleanup of leading orphan tool messages.

Testing

• Ran ruff format . and ruff check . and both succeeded.
• Attempted pytest tests/test_openai_source.py -k "pop_record" but the run failed in this environment due to a missing test dependency (pytest_asyncio) and so the test execution could not complete.
• Lint/format checks passed and the change was committed and is ready for CI to run full test suite (including the new async tests).

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Codex Task