Allow has_ai_credentials() filter fallback for non-API-key connectors

[raftaar1191]

What?

Closes #336

Adds a filter-based fallback to has_ai_credentials() so third-party/local providers that do not use API keys can still mark credentials as configured.

Why?

The current configured-state check only returns true when an ai_provider connector uses authentication.method = api_key and the related setting is non-empty. Local/self-hosted connectors (for example connectors using authentication.method = none) can be fully usable but still fail this configured check, which triggers the settings notice and blocks expected behavior.

How?

1. Cache registered connectors in $connectors = wp_get_connectors().
2. Keep existing api_key logic unchanged.
3. If no API-key credential is found, call:
   apply_filters( 'wpai_has_ai_credentials', false, $connectors )
4. Update the filter docs to reflect the second argument is the full connectors array.

Use of AI Tools

Drafting and implementation assistance was done with GitHub Copilot (GPT-5.3-Codex). All changes were reviewed and validated before submission.

Testing Instructions

1. Check out this PR branch.
2. Ensure no AI provider API key options are set.
3. Register an ai_provider connector that does not use API-key auth (for example auth method none).
4. Add a filter callback to wpai_has_ai_credentials returning true when that connector is present.
5. Verify \WordPress\AI\has_ai_credentials() returns true.
6. Verify AI settings no longer show the "requires a valid AI Connector" configured-state notice in this scenario.
7. Remove the filter and verify behavior returns to previous default (false without API-key credentials).

Testing Instructions for Keyboard

1. Open the AI settings page using keyboard navigation only.
2. Verify focus order and controls are unchanged.
3. Confirm the configured-state notice visibility changes only based on credential/filter state, with no keyboard regressions.

Screenshots or screencast

N/A (no UI/layout changes; behavior-only change).

Before	After
Configured check could not be overridden with connector context for non-API-key providers.	Configured check can be overridden via wpai_has_ai_credentials with access to registered connectors.

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Co-authored-by: raftaar1191 <raftaar1191@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.70%. Comparing base (32c5e2d) to head (4c5d711).
⚠️ Report is 17 commits behind head on develop.

Files with missing lines	Patch %	Lines
includes/helpers.php	0.00%	6 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #337      +/-   ##
=============================================
- Coverage      57.85%   57.70%   -0.15%     
- Complexity       615      617       +2     
=============================================
  Files             46       46              
  Lines           3165     3173       +8     
=============================================
  Hits            1831     1831              
- Misses          1334     1342       +8     

Flag	Coverage Δ
unit	57.70% <0.00%> (-0.15%)	⬇️

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