Preserve tool-call atomicity when popping context records and add tests

[CompilError-bts]

Motivation

• Ensure that removing old conversation records does not break the pairing between assistant tool_calls and subsequent tool messages.
• Make pop_record robust for cases with leading orphan tool messages and multi-call assistant messages.

Description

• Rewrote Provider.pop_record to remove the earliest non-system "unit" (a single message or an assistant+tool(s) group) while preserving assistant tool_calls and their following tool messages as an atomic unit.
• Added helper functions _has_tool_calls and _next_unit_bounds to detect units and compute removal bounds, and limited total removed items to keep behavior close to the previous strategy.
• Updated tests in tests/test_openai_source.py by adding async tests for pop_record behavior: test_pop_record_removes_assistant_tool_calls_with_following_tools_atomically, test_pop_record_removes_leading_orphan_tool_messages, test_pop_record_normal_messages_no_regression, test_pop_record_assistant_with_multiple_tool_calls, and test_pop_record_only_system_messages.
• Hardened existing file-URI related tests to use Path comparisons and constructed a file://localhost URI using urlparse/urlunparse to avoid platform-dependent formatting.

Testing

• Ran the modified tests/test_openai_source.py with pytest, including the new pop_record unit tests, and the updated file-URI tests; all tests passed.
• Verified no-regression for normal message popping behavior via test_pop_record_normal_messages_no_regression, which succeeded.
• Verified edge cases for orphan tool messages and multiple tool calls via their respective tests, which succeeded.

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Codex Task