feat(fim): add FIM request and response handling for Mistral AI

[mssayari]

I've implemented the Mistral Fill-in-the-Middle (FIM) Completion endpoint. This involved creating a new generic FIM architecture within Prism so that other providers can also implement FIM in the future.

Key Changes

Core FIM Architecture

• [NEW] Request.php: Data object for FIM requests.
• [NEW] Response.php: Data object for FIM responses.
• [NEW] PendingRequest.php: Fluent builder for configuring FIM requests.
• [MODIFY] Prism.php: Added fim() method.
• [MODIFY] Provider.php: Added fim() signature.

Mistral Provider Implementation

• [MODIFY] Mistral.php: Integrated the FIM handler.
• [NEW] Fim.php: Handler that interacts with Mistral's /fim/completions endpoint.

Testing

• [NEW] MistralFimTest.php: Comprehensive tests covering successful generation and rate limit metadata.
• [NEW] fim-completion-1.json: Test fixture for FIM responses.

Usage Example

use Prism\Prism\Facades\Prism;
use Prism\Prism\Enums\Provider;

$response = Prism::fim()
    ->using(Provider::Mistral, 'codestral-latest')
    ->withPrompt('def add(a, b):')
    ->withSuffix('    print("Done")')
    ->asText();

echo $response->text; // "return a+b"

Endpoint URLs:
https://docs.mistral.ai/api/endpoint/fim
https://docs.mistral.ai/capabilities/code_generation