[bug] MistralProvider throws cryptic null-check crash instead of ArgumentError when API key is missing

[RandalSchwartz]

When initializing MistralProvider without a MISTRAL_API_KEY environment variable or an explicit apiKey, creating a model crashes with a cryptic Null check operator used on a null value error instead of the clean, user-friendly ArgumentError thrown by other providers (like Google or Anthropic).

This issue was discovered while experimenting with the antigravity-cli (agy CLI) agent, and still needs repo-side verification and fixing.

---

Root Cause Analysis

In packages/dartantic_ai/lib/src/providers/mistral_provider.dart, the MistralProvider constructor omits passing apiKeyName: defaultApiKeyName to its parent Provider constructor:

  MistralProvider({String? apiKey, super.headers})
    : super(
        apiKey: apiKey ?? tryGetEnv(defaultApiKeyName),
        name: 'mistral',
        displayName: 'Mistral',
        defaultModelNames: {
          ModelKind.chat: 'mistral-medium-latest',
          ModelKind.embeddings: 'mistral-embed',
        },
        baseUrl: null,
        aliases: ['mistralai'],
        // apiKeyName: defaultApiKeyName is missing here!
      );

As a result:

1. apiKeyName evaluates to null.
2. In createChatModel(), the following validation check is completely bypassed because apiKeyName != null is false:

   if (apiKeyName != null && (apiKey == null || apiKey!.isEmpty)) {
     throw ArgumentError('$apiKeyName is required for $displayName provider');
   }

3. The method then tries to construct MistralChatModel and applies a bang operator to apiKey (which is null):

   return MistralChatModel(
     ...
     apiKey: apiKey!, // <-- Crashes here with Null check operator used on a null value
     ...
   );

---

Suggested Fix

1. Surgically fix the constructor:
   Update packages/dartantic_ai/lib/src/providers/mistral_provider.dart to specify the missing parameter:

   MistralProvider({String? apiKey, super.headers})
     : super(
         apiKey: apiKey ?? tryGetEnv(defaultApiKeyName),
         name: 'mistral',
         displayName: 'Mistral',
         defaultModelNames: {
           ModelKind.chat: 'mistral-medium-latest',
           ModelKind.embeddings: 'mistral-embed',
         },
         baseUrl: null,
         aliases: ['mistralai'],
         apiKeyName: defaultApiKeyName, // <-- Fix
       );

2. Add unit test coverage:
   Add a regression test in packages/dartantic_ai/test/provider_initialization_test.dart to ensure it resolves correctly and throws a clean ArgumentError:

   test('Mistral provider requires MISTRAL_API_KEY to create chat models', () {
     final mistralProvider = Agent.getProvider('mistral');
   
     expect(
       mistralProvider.createChatModel,
       throwsA(
         isA<ArgumentError>().having(
           (e) => e.message,
           'message',
           contains('MISTRAL_API_KEY is required'),
         ),
       ),
     );
   });