feat(jenkins-backend): regiser MCP actions for Jenkins

[MattiaDellOca]

Hey, I just made a Pull Request!

The Jenkins backend plugin exposes REST routes for:

1. Getting projects for an entity
2. Getting builds for a job
3. Getting a specific build
4. Getting build console text
5. Triggering a rebuild

These routes serve the frontend plugin. This PR makes the same capabilities available as MCP actions, so they can be consumed by Backstage AI tooling.

Note: given that the logic between routes and MCP actions is very similar, the common logic has been removed from JenkinsBuilder and moved to jenkinsService.

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
• All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
backend	workspaces/jenkins/packages/backend	none	v0.0.11
@backstage-community/plugin-jenkins-backend	workspaces/jenkins/plugins/jenkins-backend	minor	v0.27.0

[awanlin]

Hi @MattiaDellOca, there's already a PR for this - #8536. Can you perhaps collaborate with @drodil on this given his PR was submitted first? There's also more changes in this PR than I would expect to see which struck me as a bit odd.

[MattiaDellOca]

Hello @awanlin, may I ask which changes introduced by this PR do not convince you?
Also, do you think that this PR still introduce changes that might be worth to integrate into @drodil's one? If not, I can just close this as duplicate