refactor: clarify pop_record removal policy constants

[CompilError-bts]

Motivation

• The existing pop_record loop used hard-coded numeric stopping conditions that made the intended policy ("usually remove 2 records, sometimes 3 to preserve atomic units") hard to read, so the review suggested making the policy explicit and parameterized.

Description

• Introduce named constants TARGET_RECORDS = 2 and MAX_RECORDS = 3 and rewrite the removal loop to use while removed < TARGET_RECORDS and if removed > 0 and removed + next_unit_count > MAX_RECORDS: break, preserving existing behavior while improving readability (astrbot/core/provider/provider.py).

Testing

• Ran ruff format . and ruff check ., and both checks passed.

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Codex Task