Fix server startup failure when client-ca is empty or not provided

[jianzhangbjz]

Description of the change:
This PR fixes a bug in the pkg/lib/server package where the server fails to start when TLS is enabled but the --client-ca flag is empty or not provided.

Motivation for the change:
When --client-ca is empty or nil, filemonitor.NewCertPoolStore() attempts to call os.ReadFile("") which fails with an error, causing the server to crash on startup.

This issue was discovered during code review of PR #1190 (OPRUN-4416: Remove kube-rbac-proxy from PSM), which would be affected by this bug when PSM is started without a client CA configuration.

Architectural changes:

1. Added clientCAEnabled() helper method to check if client CA path is provided and non-empty
2. Modified getListenAndServeFunc() to conditionally setup client CA monitoring only when a valid path is provided
3. Updated TLS configuration to only set ClientCAs and ClientAuth when client CA is available
4. Added certPoolGetter interface for type-safe handling of the cert pool store

Testing remarks:

Reviewer Checklist

• Implementation matches the proposed design, or proposal is updated to match implementation
• Sufficient unit test coverage
• Sufficient end-to-end test coverage
• Bug fixes are accompanied by regression test(s)
• e2e tests and flake fixes are accompanied evidence of flake testing, e.g. executing the test 100(0) times
• tech debt/todo is accompanied by issue link(s) in comments in the surrounding code
• Tests are comprehensible, e.g. Ginkgo DSL is being used appropriately
• Docs updated or added to /doc
• Commit messages sensible and descriptive
• Tests marked as [FLAKE] are truly flaky and have an issue
• Code is properly formatted

Assisted-By: Claude-Code

[tmshort]

/approve

[rashmigottipati]

/lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: grokspawn, rashmigottipati, tmshort

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [grokspawn,tmshort]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment