Improve changelog bundle --output

[lcawl]

Summary

Updated the docs-builder changelog bundle command's --output option and updated documentation and tests.

Impetus

While testing #2452 I encountered problems specifying an output directory for the bundle since it was interpreting part of the output path as a filename.

Changes Made:

• Command Implementation (src/tooling/docs-builder/Commands/ChangelogCommand.cs):
  • Updated --output to accept:
    • Directory path: creates changelog-bundle.yaml in that directory
    • File path ending in .yml or .yaml: uses that exact path
  • Added validation: if a filename is provided, it must end in .yml or .yaml
  • Updated XML documentation comment
• Documentation (docs/contribute/changelog.md and docs/cli/release/changelog-bundle.md):
  • Updated the --output option description
  • Added "Output file location" section with examples for both formats
  • Fixed code fence formatting
• Tests (tests/Elastic.Documentation.Services.Tests/ChangelogServiceTests.cs):
  • Added BundleChangelogs_WithDirectoryOutputPath_CreatesDefaultFilename to verify directory-only output paths work correctly

All 31 bundle-related tests pass

Behavior

• --output /path/to/dir → creates /path/to/dir/changelog-bundle.yaml
• --output /path/to/file.yaml → creates /path/to/file.yaml
• --output /path/to/file.txt → error (must end in .yml or .yaml)

The implementation is backward compatible: existing tests continue to pass, and the service layer already handles directory creation.

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

4. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: composer-1 agent