Add Mistral as AI provider

- Add Mistral to KnownProvider type and model generation
- Implement Mistral-specific compat handling in openai-completions:
  - requiresToolResultName: tool results need name field
  - requiresAssistantAfterToolResult: synthetic assistant message between tool/user
  - requiresThinkingAsText: thinking blocks as <thinking> text
  - requiresMistralToolIds: tool IDs must be exactly 9 alphanumeric chars
- Add MISTRAL_API_KEY environment variable support
- Add Mistral tests across all test files
- Update documentation (README, CHANGELOG) for both ai and coding-agent packages
- Remove client IDs from gemini.md, reference upstream source instead

Closes #165

packages/ai/test/empty.test.ts

@@ -289,4 +289,24 @@ describe("AI Providers Empty Message Tests", () => {
 			await testEmptyAssistantMessage(llm);
 		});
 	});
+
+	describe.skipIf(!process.env.MISTRAL_API_KEY)("Mistral Provider Empty Messages", () => {
+		const llm = getModel("mistral", "devstral-medium-latest");
+
+		it("should handle empty content array", async () => {
+			await testEmptyMessage(llm);
+		});
+
+		it("should handle empty string content", async () => {
+			await testEmptyStringMessage(llm);
+		});
+
+		it("should handle whitespace-only content", async () => {
+			await testWhitespaceOnlyMessage(llm);
+		});
+
+		it("should handle empty assistant message in conversation", async () => {
+			await testEmptyAssistantMessage(llm);
+		});
+	});
 });