Include diffs in JSON listings and mutants.json

[Copilot]

• [x] Analyze the repository structure and understand the issue
• [x] Understand how mutant JSON serialization currently works
• [x] Identify that diffs are only included in JSON when --diff flag is used
• [x] Modify list_mutants to always include diff in JSON output
• [x] Update test snapshots to reflect the new behavior
• [x] Run cargo fmt and cargo test
• [x] Verify the change works correctly with manual testing
• [x] Update documentation (book/src/list.md and NEWS.md)
• [x] Include diffs in mutants.out/mutants.json written during test runs
• [x] Add test to verify mutants.out/mutants.json includes diffs
• [x] Refactor common code into Mutant::to_json() and mutants_to_json_string()
• [x] Fold test logic into existing uncaught_mutant_in_factorial test

Summary

Changed the JSON output behavior to always include diffs in:

1. --list --json output
2. mutants.out/mutants.json written during test runs

This makes the JSON output more complete and useful for programmatic consumers.

Final Changes:

1. Added Mutant::to_json() method that includes the diff field
2. Added mutants_to_json_string() helper function in src/list.rs
3. Refactored both list_mutants() and write_mutants_list() to use the common code
4. Folded test validation into existing uncaught_mutant_in_factorial test to save test time
5. Updated test snapshots to reflect the new behavior
6. Updated documentation in book/src/list.md and NEWS.md

Testing:

• All existing tests pass (24 list tests, 50 main tests)
• Diff validation now part of existing test that runs on the factorial tree
• Manual testing confirms JSON output includes diffs by default
• Text output behavior remains unchanged (still requires --diff flag)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Include the diff as a text field within the json representation of mutants</issue_title> <issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes sourcefrog/cargo-mutants#563

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[sourcefrog]

@copilot Please check that the diff is also included in mutants.out/mutants.json written when running the mutant tests. It looks like it's only in the list output. Add one test to check that it's in there too.

[Copilot]

@copilot Please check that the diff is also included in mutants.out/mutants.json written when running the mutant tests. It looks like it's only in the list output. Add one test to check that it's in...

Done! The diff is now included in mutants.out/mutants.json. I've:

1. Modified write_mutants_list() in src/output.rs to include diffs when serializing mutants
2. Added test mutants_json_includes_diffs that verifies all mutants in mutants.out/mutants.json have a diff field with valid unified diff content
3. Updated the snapshot for the existing uncaught_mutant_in_factorial test to reflect the new format

Commit: 9b0ac85