fix(cloudflare): preserve passthrough image service in compile mode

[adamchal]

Changes

• Gives passthroughImageService() precedence when using imageService: 'compile', fixing image requests during development and on-demand rendering.
• Adds a warning when this overrides an explicitly configured runtime: 'cloudflare-binding'.

Testing

• Adds test to cover passthrough precedence when combined with 'compile'.

Docs

N/a

Closes #17968

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[adamchal]

@matthewp My concern with #18031 is that it overrides behavior and warns for all custom image services. This PR targets passthroughImageService() with 'compile' specifically.

The warning runs in astro:build:start, after integrations have finalized the image service. This catches an integration that introduces passthrough alongside an explicit runtime: 'cloudflare-binding'.

Endpoint selection still happens early. That means an integration-added passthrough service can miss the override: the build warning can detect the conflicting runtime configuration, but it doesn’t fix endpoint selection in that case. Dev requests—and production requests when using the binding runtime—can still fail.

I considered appending a separate integration to apply the override later, but that doesn’t guarantee ordering either. I’ve kept this fix scoped to directly configured passthrough rather than expanding it into a general solution for integration ordering.

The added tests cover passthrough precedence in dev and build. I didn’t add separate warning coverage. The focus is the endpoint behavior.

[matthewp]

Sounds good, I'll close that one.