Add URI validation for EventType Source and Schema fields

[Kunal1522]

Changes

Fixes #8836
This PR addresses the TODO comments in eventtype_validation.go by adding validation for the Source and Schema URI fields in EventTypeSpec.
Before: An EventType could be created with empty Source/Schema URIs (e.g., "source": ""), which would parse successfully but result in a meaningless empty URI.
After: If Source or Schema is provided (non-nil) but empty, validation will reject it with a clear error message.

What type of PR is this?

/kind cleanup

What this PR does

• Adds validation in EventTypeSpec.Validate() to check that Source and Schema fields are not empty when provided
• Adds corresponding test cases for empty URI validation

Which issue(s) this PR fixes

Fixes the following TODOs in the codebase:

• // TODO validate Source is a valid URI.
• // TODO validate Schema is a valid URI.

How was this tested?

go test -v ./pkg/apis/eventing/v1beta1/... -run TestEventType

[knative-prow]

Hi @Kunal1522. Thanks for your PR.

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[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Kunal1522
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[creydr]

/ok-to-test

[creydr]

/lgtm