SeparateApplicationYamlByProfile: merge into existing profile files

[Jenson3210]

Summary

• SeparateApplicationYamlByProfile now merges extracted profile content into existing application-{profile}.yml files instead of generating duplicates
• Added tests for the merge scenario and for leaving unrelated existing profile files untouched

Problem

When application.yml contains a profile section (e.g. spring.config.activate.on-profile: dev) and application-dev.yml already exists, the recipe's generate() method creates a new source file with the same path. Since the framework tracks files by UUID rather than path, this results in a duplicate file that can overwrite the existing one when applied.

Solution

1. Scanner: Now also detects existing application-*.yml files and records their paths
2. Generate: Only creates new files for profiles where no target file exists; queues documents for merging when target files already exist
3. Visitor: Appends extracted profile documents (with --- separator) to existing profile files

Test plan

• Existing tests pass (separateProfile, separateProfileList, leaveProfileExpressionsAlone)
• New test: mergeIntoExistingProfileFile — verifies content is appended to existing profile file
• New test: doNotModifyExistingProfileFileWhenNoMatchingProfile — verifies unrelated profile files are untouched