feat: add streaming support and new GPT-Image models for OpenAI

[apappascs]

Add support for OpenAI's new GPT-Image models (gpt-image-1 and gpt-image-1-mini) with streaming image generation capabilities and all new model-specific parameters according to the official API specification.

Changes:

Core Interface & API Layer:

• Add StreamingImageModel interface to spring-ai-model for progressive image generation
• Implement streamImage() method in OpenAiImageApi using WebClient for SSE support
• Add OpenAiImageStreamEvent record to handle streaming events (partial_image and completed)
• Add dynamic API key resolution pattern to fix test failures
• Add GPT_IMAGE_1 and GPT_IMAGE_1_MINI to ImageModel enum
• Update OpenAiImageApi constructor to accept WebClient.Builder for streaming support

Model Layer:

• Implement StreamingImageModel interface in OpenAiImageModel
• Add stream() method with automatic stream parameter setting
• Add validation to restrict streaming to GPT-Image models only
• Add comprehensive error handling for non-streaming models

Parameters & Options:

• Add 6 new gpt-image-1 specific parameters to OpenAiImageRequest and OpenAiImageOptions:
  • background: transparency control (transparent/opaque/auto)
  • moderation: content moderation level (low/auto)
  • outputCompression: compression level 0-100% for webp/jpeg
  • outputFormat: output format (png/jpeg/webp)
  • partialImages: streaming partial images support (0-3, documented limit)
  • stream: enable streaming mode
• Add size() builder method to OpenAiImageOptions.Builder
• Update OpenAiImageOptions with new fields, getters/setters, and builder methods

Testing:

• Add OpenAiImageApiStreamingIT with 4 comprehensive streaming tests
• Add OpenAiImageModelStreamingIT with 6 end-to-end streaming tests
• Update OpenAiImageApiIT with tests for all models and new parameters
• Add parameterized tests for all ImageModel enum values
• Fix partialImages parameter values to respect ≤3 limit
• Update observation tests to use GPT_IMAGE_1_MINI

Auto-configuration & CI:

• Update OpenAiImageAutoConfiguration to inject WebClient.Builder
• Update .github/workflows/pr-check.yml to build spring-ai-model first
• Add separate test jobs for API and Model integration tests
• Add reactor-test dependency to pom.xml for streaming tests

Documentation:

• Add StreamingImageModel interface documentation to imageclient.adoc
• Document streaming support and limitations in openai-image.adoc
• Add comprehensive streaming examples with partial images
• Document all new parameters with model-specific constraints
• Clarify that DALL-E models do not support streaming

Reference: https://platform.openai.com/docs/models
https://platform.openai.com/docs/api-reference/images-streaming
https://platform.openai.com/docs/guides/image-generation?image-generation-model=gpt-image-1&api=image