feat: migrate xAI provider to Responses API with reusable transform utilities

Migrate the xAI provider from the deprecated Chat Completions API to the
Responses API, add grok-4.20 as the new default model, and introduce shared
transform utilities for Responses API that other providers can adopt.

New shared utilities (src/api/transform/):
- responses-api-stream.ts: processResponsesApiStream() handles core
  Responses API stream events (text, reasoning, tool calls, usage) and
  createUsageNormalizer() provides configurable token/cost extraction.
  Designed for reuse by openai-native, openai-codex, or any future
  Responses API provider.
- responses-api-input.ts: convertToResponsesApiInput() converts directly
  from Anthropic message format to Responses API input format, avoiding
  the intermediate Chat Completions conversion step. Handles input_text,
  input_image, function_call, and function_call_output mappings.

xAI provider (src/api/providers/xai.ts):
- Switch from client.chat.completions.create() to client.responses.create()
- Use shared transform utilities for stream handling and input conversion
- Enable reasoning traces via include: ["reasoning.encrypted_content"]
- System prompt via instructions field, store: false for privacy
- completePrompt() also migrated to Responses API

Model updates (packages/types/src/providers/xai.ts):
- Add grok-4.20 as the new default model (2M context, $2/$6 pricing)
- Remove grok-4.20-beta-0309-reasoning and grok-4.20-beta-0309-non-reasoning

Author: Enrico Carlesso

packages/types/src/providers/xai.ts

@@ -3,10 +3,10 @@ import type { ModelInfo } from "../model.js"
 // https://docs.x.ai/docs/api-reference
 export type XAIModelId = keyof typeof xaiModels
 
-export const xaiDefaultModelId: XAIModelId = "grok-4.20-beta-0309-reasoning"
+export const xaiDefaultModelId: XAIModelId = "grok-4.20"
 
 export const xaiModels = {
-	"grok-4.20-beta-0309-reasoning": {
+	"grok-4.20": {
 		maxTokens: 65_536,
 		contextWindow: 2_000_000,
 		supportsImages: true,
@@ -15,21 +15,7 @@ export const xaiModels = {
 		outputPrice: 6.0,
 		cacheWritesPrice: 0.5,
 		cacheReadsPrice: 0.5,
-		description:
-			"xAI's Grok 4.20 reasoning model with 2M context. Reasoning is internal (not exposed via Chat Completions API).",
-		includedTools: ["search_replace"],
-		excludedTools: ["apply_diff"],
-	},
-	"grok-4.20-beta-0309-non-reasoning": {
-		maxTokens: 65_536,
-		contextWindow: 2_000_000,
-		supportsImages: true,
-		supportsPromptCache: true,
-		inputPrice: 2.0,
-		outputPrice: 6.0,
-		cacheWritesPrice: 0.5,
-		cacheReadsPrice: 0.5,
-		description: "xAI's Grok 4.20 non-reasoning model - faster inference with 2M context.",
+		description: "xAI's flagship Grok 4.20 model with 2M context and reasoning support via Responses API.",
 		includedTools: ["search_replace"],
 		excludedTools: ["apply_diff"],
 	},