test: improve policy exception validation coverage

[karthikmanam]

Explanation

Improve the PolicyException validation test coverage by adding missing validation scenarios and making the GetKind subtest names more descriptive. This is a test-only change and does not modify production behavior.

Related issue

Fixes #104

Proposed Changes

• Rename the duplicate TestCELPolicyException_GetKind subtest for clearer test output.
• Add validation test coverage for a policy reference missing both name and kind.
• Add validation test coverage for multiple invalid policy references with indexed field errors.
• Remove the existing TODO by covering the previously untested cases.

Checklist

• I have read the contributing guidelines.
• I have read the PR documentation guide and followed the process including adding proof manifests to this PR.
• This is a bug fix and I have added unit tests that prove my fix is effective.

Further Comments