test: cover config migration dry run

[theamodhshetty]

Summary

• add focused tests for ConfigSourceMigration.dry_run() output
• verify dry runs leave the config untouched for rename, removal, unset, and missing-key paths

Tests

• .venv/bin/ruff format tests/config/test_config_source.py
• .venv/bin/ruff check tests/config/test_config_source.py
• .venv/bin/python -m pytest tests/config/test_config_source.py -n 0

Relates-to: #3155

[sourcery-ai]

Hey - I've left some high level feedback:

• The dry-run tests hard-code the full io.fetch_output() string including spacing and newlines, which may be brittle; consider normalizing (e.g., using .strip() or checking substrings) so minor formatting changes don't unnecessarily break the tests.
• The four dry_run tests share a lot of setup and follow the same pattern; you could reduce duplication and make future changes easier by parametrizing them (e.g., expected return value and expected output per migration scenario).

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The dry-run tests hard-code the full `io.fetch_output()` string including spacing and newlines, which may be brittle; consider normalizing (e.g., using `.strip()` or checking substrings) so minor formatting changes don't unnecessarily break the tests.
- The four `dry_run` tests share a lot of setup and follow the same pattern; you could reduce duplication and make future changes easier by parametrizing them (e.g., expected return value and expected output per migration scenario).

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[theamodhshetty]

Thanks for the review. I pushed bd622ac to parametrize the dry-run cases and trim the trailing newline from the output comparison, while still keeping the actual user-facing message text asserted.

Local checks I reran:

• .venv/bin/ruff format tests/config/test_config_source.py
• .venv/bin/ruff check tests/config/test_config_source.py
• .venv/bin/python -m pytest tests/config/test_config_source.py -n 0

The earlier Ubuntu 3.10 pytest failure looks unrelated to this diff (sys.platform was mocked as darwin in an installer test and leaked into sysconfig). The new push should give CI a fresh run.