Support local AI providers without API keys in credential checks

[raftaar1191]

Description

When only a local/self-hosted AI provider is installed (no API key required), the AI settings page incorrectly reports that no valid AI connector is configured.

Expected behavior:

• Local providers that do not require API-key auth should be able to work with the AI plugin.

Current behavior:

• The AI settings page shows: "The AI plugin requires a valid AI Connector to function properly..." even when a local provider plugin is installed and configured.

Step-by-step reproduction instructions

1. Install WordPress 7.0 + AI plugin (v0.6.0).
2. Install and activate a local provider plugin, for example:
   • https://github.com/Fueled/ai-provider-for-ollama/
3. Do not configure any cloud API key connector.
4. Open wp-admin/options-general.php?page=ai.
5. Observe the error notice that says no valid AI connector is configured.

Screenshots, screen recording, code snippet

Error text shown in settings page:

The AI plugin requires a valid AI Connector to function properly. Verify you have one or more AI Connectors configured here.

Context:

• Local-hosted providers such as Ollama/llama.cpp-style integrations may not need API credentials.

Environment info

• WordPress version: 7.0
• AI plugin version: 0.6.0
• Local provider example: ai-provider-for-ollama
• Theme: not relevant to reproduce (tested on admin settings page)
• Device/OS: macOS desktop
• Browser: Chrome (also reproducible in other browsers)

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except the AI plugin.

• Yes (exception: local provider plugin required to reproduce)

Please confirm which theme type you used for testing.

• Not sure

Additional note

A filter-based extension point in has_ai_credentials() would allow third-party local providers to declare credential availability for non-API-key connectors while preserving default behavior for API-key connectors.