pkg/alertmanager: add URL validation checks for Incident.io receiver

[tomlin7]

Description

This PR adds test coverage to verify URL validation logic for the Incidentio receiver in Alertmanager configuration. The validation ensures that the url field, when provided, is a well-formed URL.

Relates to #8193

Type of change

What type of changes does your code introduce to the Prometheus operator? Put an x in the box that apply.

• CHANGE (fix or feature that would cause existing functionality to not work as expected)
• FEATURE (non-breaking change which adds functionality)
• BUGFIX (non-breaking change which fixes an issue)
• ENHANCEMENT (non-breaking change which improves existing functionality)
• NONE (if none of the other choices apply. Example, tooling, build system, CI, docs, etc.)

Verification

• Ran tests: go test ./pkg/alertmanager -run TestSanitizeIncidentioConfig
• Verified that invalid URLs return an appropriate error.

Changelog entry

Please put a one-line changelog entry below. This will be copied to the changelog file during the release process.

Add URL validation for Incident.io receiver secrets