feat(fireworks): add FireworksImageModelOptions type with providerOptions validation

[saakshigupta2002]

Summary

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

This addresses the lack of type safety for Fireworks image generation provider options. Previously, providerOptions.fireworks was spread directly into the request body without any validation.

Changes

• packages/fireworks/src/fireworks-image-options.ts: Added Zod schema (fireworksImageModelOptions) defining all supported Fireworks image generation options (cfg_scale, steps, negative_prompt, strength, scheduler, safety_checker, output_format, safety_tolerance). Exported inferred FireworksImageModelOptions type.
• packages/fireworks/src/fireworks-image-model.ts: Integrated parseProviderOptions to validate provider options against the Zod schema at runtime, replacing unvalidated ...(providerOptions.fireworks ?? {}) spread. Provider options are now consumed via individual property assignments.
• packages/fireworks/src/fireworks-image-model.test.ts: Updated tests to use schema-defined options instead of arbitrary pass-through properties.
• packages/fireworks/src/index.ts: Exported FireworksImageModelOptions type.
• examples/ai-functions/src/generate-image/fireworks.ts: Updated example to import and use satisfies FireworksImageModelOptions for type-safe provider options.

Verification

• Build passes (pnpm turbo build --filter=@ai-sdk/fireworks)
• Lint passes (pnpm turbo lint --filter=@ai-sdk/fireworks)
• All existing tests pass (33/33)

Closes #12439