Add Bedrock image provider support

[jaksa-v]

Summary

• expose Bedrock image generation through the Laravel AI provider bridge
• add Bedrock image model defaults and image-specific request options
• return the Bedrock provider from Ai::imageProvider('bedrock')

Why

clinically/prism-bedrock already supports Bedrock image generation, but this package does not expose that capability through Laravel AI. As a result, applications have to drop below the Laravel AI layer just to generate Bedrock images.

What changed

• Made BedrockProvider implement ImageProvider and added the corresponding image gateway traits.
• Added image model defaults and Bedrock image option mapping in the package config and provider.
• Implemented BedrockPrismGateway::generateImage() for the Bedrock driver.
• Added tests covering image provider registration and Titan image option normalization.
• Updated the Bedrock service provider namespace in tests so the branch remains independently runnable against the current upstream baseline.

Review note

The only overlap with #1 is the minimal Bedrock service provider namespace fix in the tests. This PR does not include the separate Titan embeddings bridge or provider-option merge work.

Verification

• php vendor/bin/phpunit