Enhance changelog add command with github release workflow

[lcawl]

Summary

Per #2841, the docs-builder changelog gh-release command creates both changelog files and a bundle from a GitHub release. This PR enhances the docs-builder changelog add command to also be able to do just the first half of that functionality, in case teams want to separate the tasks. For example, maybe there's a case where they want to create all the changelogs but bundle later (or use exceptional bundle commands). (The use case here isn't as obvious as the upcoming "bundle" one, but IMO it doesn't hurt to have both split out).

Files changed

• src/services/Elastic.Changelog/GithubRelease/GitHubReleaseChangelogService.cs — added CreateBundle property to CreateChangelogsFromReleaseArguments (defaults true); bundle creation is now gated on it

• src/tooling/docs-builder/Commands/ChangelogCommand.cs — added --release-version parameter to changelog add; when set, validates mutual exclusivity with --prs/--issues and requirement of --repo, then delegates to GitHubReleaseChangelogService with CreateBundle = false

• docs/cli/release/changelog-add.md — documented --release-version option and updated --repo description

• docs/contribute/changelog.md — added "Create changelogs from a release" subsection with example; updated Authorization note to include --release-version

• tests/Elastic.Changelog.Tests/Changelogs/Create/ReleaseVersionTests.cs (new) — 6 tests covering: no PR refs warning, changelog files created without bundle, bundle created when CreateBundle = true, latest tag pass-through, fetch failure error, and unknown repo error

All 244 tests pass.

Generative AI disclosure

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

• Yes
• No

2. 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-1.5, clause-4.6-sonnet-medium

[github-actions]

🔍 Preview links for changed docs

• docs/cli/release/changelog-add.md
• docs/contribute/changelog.md