feat(amazon-bedrock): add AmazonBedrockImageModelOptions type with providerOptions validation

[saakshigupta2002]

Summary

Adds a proper AmazonBedrockImageModelOptions type definition for Amazon Bedrock image model provider options, backed by a Zod schema and validated at runtime using parseProviderOptions.

This addresses the lack of type safety for Bedrock image generation provider options. Previously, users had no TypeScript guidance when passing providerOptions.bedrock for image models.

Changes

• packages/amazon-bedrock/src/bedrock-image-settings.ts: Added Zod schema (amazonBedrockImageModelOptions) defining all supported Bedrock image generation options (quality, cfgScale, negativeText, style, taskType, maskPrompt, outPaintingMode, similarityStrength). Exported inferred AmazonBedrockImageModelOptions type.
• packages/amazon-bedrock/src/bedrock-image-model.ts: Integrated parseProviderOptions to validate provider options against the Zod schema at runtime, replacing unvalidated raw providerOptions.bedrock access. Provider options are now consumed via individual property assignments.
• packages/amazon-bedrock/src/index.ts: Exported AmazonBedrockImageModelOptions type.
• examples/ai-functions/src/generate-image/amazon-bedrock.ts: Updated example to import and use satisfies AmazonBedrockImageModelOptions for type-safe provider options.

Verification

• Build passes (pnpm turbo build --filter=@ai-sdk/amazon-bedrock)
• Lint passes (pnpm turbo lint --filter=@ai-sdk/amazon-bedrock)
• All existing tests pass (25/25)

Closes #12435