Fix[Image Generation]: Detect support for connectors that authenticate without an API key (e.g. OAuth)

[diarmuidie]

What?

Follow up to #679.

has_image_generation_support() skipped connectors that authenticate without an API key (e.g. OAuth) because it only checked for API-key credentials. It now also honors connectors the AI client registry reports as configured.

Why?

#679 gates the image generation UI on has_image_generation_support(). That check relied solely on has_connector_authentication(), which by design only detects API-key credentials (environment variable, constant, or stored option).

As a result, a provider that is fully configured but authenticates via OAuth was never counted as supporting image generation, and the image generation UI stayed hidden even though the provider exposes an image-generation-capable model.

This mirrors how text generation credential detection already honors connectors that do not rely on API key settings.

How?

In the connector loop, a connector now qualifies when it either has API-key credentials or is_connector_configured() reports it as configured through the registry:

if ( ! has_connector_authentication( $connector_id ) && ! is_connector_configured( $connector_id ) ) {
    continue;
}

A $reset_cache parameter was added to has_image_generation_support() so the per-request memoization can be recomputed (used by the new tests, and useful when connector configuration changes within a single request).

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 4.8
Used for: Investigating the regression, drafting the fix, and writing the integration tests; reviewed and edited by me.

Testing Instructions

Automated: tests/Integration/Includes/HelpersTest.php covers has_image_generation_support() for the API-key path, the non-API-key (registry-configured) path, the missing-capability case, and the unauthenticated/unconfigured skip path.

Manual:

1. Configure an AI connector that authenticates without an API key (e.g. OAuth) and exposes an image-generation-capable model, with no API key set.
2. Verify the image generation UI is now available (it was hidden before this change).
3. Regression check: an API-key connector with image support still shows the UI; no configured/authenticated connectors keeps it hidden.

Changelog Entry

Fixed - Image generation is now detected for connectors that authenticate without an API key (e.g. OAuth).

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