FIX: Disallow Empty robot_name_prefix to prevent OIDC CLI login from being blocked

[falconlee236]

What

• Validate robot_name_prefix as non-empty so and empty/whitespace-only prefix cannot be saved.

Why

• With an empty prefix, the OIDC CLI middleware treats and every username as a robot account due to string.HasPrefix(username, prefix) always being true

How

Thanks to @stonezdj, I found clue how to handle this issue.

• Change config metadata for robot_name_prefix from StringType to NonEmptyStringType`
• Server-side validation already flows through metadata.NewCfgValue(...) -> ConfigureValue.Set(...) -> ItemType.Validate(...). so empty/space-only values now return ErrStringValueIsEmpty and are rejected during config update.

Issue being fixed

Fixes #22395

Please indicate you've done the following:

• Well Written Title and Summary of the PR
• Label the PR as needed. "release-note/ignore-for-release, release-note/new-feature, release-note/update, release-note/enhancement, release-note/community, release-note/breaking-change, release-note/docs, release-note/infra, release-note/deprecation"
• Accepted the DCO. Commits without the DCO will delay acceptance.
• Made sure tests are passing and test coverage is added if needed.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed in website repository.

[stonezdj]

LGTM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.86%. Comparing base (c8c11b4) to head (c3ef1e1).
⚠️ Report is 594 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #22556       +/-   ##
===========================================
+ Coverage   45.36%   65.86%   +20.49%     
===========================================
  Files         244     1073      +829     
  Lines       13333   116095   +102762     
  Branches     2719     2931      +212     
===========================================
+ Hits         6049    76470    +70421     
- Misses       6983    35388    +28405     
- Partials      301     4237     +3936     

Flag	Coverage Δ
unittests	65.86% <ø> (+20.49%)	⬆️

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