Flatten Mistral image payload to use direct image_url string

[Android-PowerUser]

Motivation

• Simplify the Mistral multimodal request shape by sending the image URL/base64 directly as a string instead of wrapping it in an extra MistralImageUrl object.

Description

• Replaced MistralImageUrl / ServiceMistralImageUrl wrapper types with a plain String for the image_url field in MistralImageContent and ServiceMistralImageContent.
• Updated all call sites to pass the data:image/jpeg;base64,... string directly when constructing image content payloads in ScreenCaptureApiClients.kt and PhotoReasoningViewModel.kt.
• Removed the now-unused MistralImageUrl/ServiceMistralImageUrl data classes and adjusted serializer polymorphic registrations to match the new shape.

Testing

• Built the app with ./gradlew :app:assemble to ensure serialization and compilation succeed, and the build completed successfully.
• Ran unit tests with ./gradlew test and all tests passed.

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Codex Task

[amazon-q-developer]

The changes in this PR successfully simplify the Mistral multimodal request structure by removing the unnecessary MistralImageUrl wrapper and using direct string values for image URLs. The refactoring is consistently applied across all three files, maintains proper serialization annotations, and preserves the existing functionality. The flattened structure reduces nesting and improves code clarity without introducing any defects.

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