Detect unified diffs in whole-file mode

[adbcodes]

Fixes #5486

Summary

• detect unified-diff-shaped responses when whole-file parsing produces no edits
• report the mismatch through Aider's existing malformed-response path
• avoid automatically applying an unexpected edit format
• provide actionable retry and configuration guidance
• add regression and no-change boundary tests

Testing

• pytest tests/basic/test_wholefile.py -q
• pytest tests/basic/test_coder.py -q
• pre-commit run --all-files

[CLAassistant]

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