Fix crash when creating an existing patchinfo package

[Sahitya0805]

Hey Friends,

this PR resolves a crash in the Patchinfo creation flow when a package with the name "patchinfo" already exists in the project. Instead of throwing an ActiveRecord::RecordInvalid exception, the controller now catches it and shows a user-friendly error message.

Expected Behavior:

• If a "patchinfo" package already exists, the system should not crash.
• The user should be redirected back gracefully.
• A clear flash error message should be displayed indicating that the package already exists.

To verify this change:

• Go to a project where you have permissions (e.g., your home project).
• Create a package named patchinfo manually if it doesn't exist.
• Click on "Create patchinfo" in the project actions.
• You should be redirected back to the project page with a flash error: "Validation failed: Project ... already has a package with the name patchinfo".

Automated tests:

Run the regression test to ensure the fix works as expected:

bundle exec rspec spec/controllers/webui/patchinfo_controller_spec.rb:114

Detailed Changes:

• Modified PatchinfoController#create to rescue ActiveRecord::RecordInvalid.
• Added a regression test in patchinfo_controller_spec.rb to cover the validation failure scenario.

Fixes #18711