Fail fast with clear error when Jenkins root URL is missing

[PiyushMalik01]

Fixes #114

What this PR does

When the Jenkins root URL is not configured, MCP tool calls currently fail with an unclear
serialization error that does not explain the real problem.

This PR adds an explicit check for a missing Jenkins root URL and returns a clear,
actionable error message that guides the user to fix the issue via
Manage Jenkins → Configure System.

This makes the failure easier to understand and avoids confusing low-level errors.

---

Testing done

Manually tested on a local Jenkins instance started with mvn hpi:run.

• Verified that calling an MCP tool without configuring the Jenkins root URL now returns a clear, user-friendly error message
• Verified that MCP tools continue to work as expected once the root URL is configured

No automated tests were added since this behavior depends on Jenkins runtime configuration.

---

Submitter checklist

• Ensure that the pull request title represents the desired changelog entry
• Describe what was changed
• Manually tested the change

[topikachu]

Could you move this validation logic to the beginning of Endpoint#init so it fails fast during initialization?

Also, since this depends on the Jenkins root URL being properly configured, we should explicitly document that requirement in the README and explain how to set it.

Thanks

[PiyushMalik01]

ok , I’ll push an update shortly

[PiyushMalik01]

@topikachu
Thanks for the feedback! I've updated the implementation to validate the Jenkins root URL at the beginning of tool invocation, so it fails early instead of during result serialization. Please let me know if this looks good.

[PiyushMalik01]

@topikachu I’ve added a section to the README documenting the Jenkins root URL requirement and how to configure it. Please let me know if the placement or wording looks good.