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/src/utils/overflow.ts

@@ -17,6 +17,7 @@ import type { AssistantMessage } from "../types.js";
  * - llama.cpp: "the request exceeds the available context size, try increasing it"
  * - LM Studio: "tokens to keep from the initial prompt is greater than the context length"
  * - Cerebras: Returns "400 status code (no body)" - handled separately below
+ * - Mistral: Returns "400 status code (no body)" - handled separately below
  * - z.ai: Does NOT error, accepts overflow silently - handled via usage.input > contextWindow
  * - Ollama: Silently truncates input - not detectable via error message
  */
@@ -52,6 +53,7 @@ const OVERFLOW_PATTERNS = [
  * - xAI (Grok): "maximum prompt length is X but request contains Y"
  * - Groq: "reduce the length of the messages"
  * - Cerebras: 400/413 status code (no body)
+ * - Mistral: 400/413 status code (no body)
  * - OpenRouter (all backends): "maximum context length is X tokens"
  * - llama.cpp: "exceeds the available context size"
  * - LM Studio: "greater than the context length"
@@ -85,7 +87,7 @@ export function isContextOverflow(message: AssistantMessage, contextWindow?: num
 			return true;
 		}
 
-		// Cerebras returns 400/413 with no body - check for status code pattern
+		// Cerebras and Mistral return 400/413 with no body - check for status code pattern
 		if (/^4(00|13)\s*(status code)?\s*\(no body\)/i.test(message.errorMessage)) {
 			return true;
 		}