feat: support 1M context window via anthropic_beta header for Bedrock

When Claude Code sends a model with [1m] suffix (e.g. claude-opus-4-6[1m]),
the relay now adds 'context-1m-2025-08-07' to the anthropic_beta array in
the Bedrock request body. This enables the 1M context window preview on
Bedrock for supported models (Opus 4.6, Sonnet 4.5, Sonnet 4).

Per Anthropic docs: the 1M context requires the beta header on all
platforms including Amazon Bedrock.

src/services/bedrockRelayService.js

@@ -136,12 +136,12 @@ class BedrockRelayService {
         }
       }
 
-      const modelId = this._selectModel(requestBody, bedrockAccount)
+      const { modelId, isLongContext } = this._selectModel(requestBody, bedrockAccount)
       const region = this._selectRegion(modelId, bedrockAccount)
       const client = this._getBedrockClient(region, bedrockAccount)
 
       // 转换请求格式为Bedrock格式
-      const bedrockPayload = this._convertToBedrockFormat(requestBody)
+      const bedrockPayload = this._convertToBedrockFormat(requestBody, { isLongContext })
 
       const command = new InvokeModelCommand({
         modelId,
@@ -277,12 +277,12 @@ class BedrockRelayService {
         }
       }
 
-      const modelId = this._selectModel(requestBody, bedrockAccount)
+      const { modelId, isLongContext } = this._selectModel(requestBody, bedrockAccount)
       const region = this._selectRegion(modelId, bedrockAccount)
       const client = this._getBedrockClient(region, bedrockAccount)
 
       // 转换请求格式为Bedrock格式
-      const bedrockPayload = this._convertToBedrockFormat(requestBody)
+      const bedrockPayload = this._convertToBedrockFormat(requestBody, { isLongContext })
 
       const command = new InvokeModelWithResponseStreamCommand({
         modelId,
@@ -399,6 +399,7 @@ class BedrockRelayService {
   }
 
   // 选择使用的模型
+  // Returns { modelId, isLongContext }
   _selectModel(requestBody, bedrockAccount) {
     let selectedModel
 
@@ -420,15 +421,18 @@ class BedrockRelayService {
       logger.info(`🎯 使用系统默认模型: ${selectedModel}`, { metadata: { source: 'default' } })
     }
 
+    // Detect [1m] long context variant before mapping
+    const isLongContext = selectedModel.includes('[1m]')
+
     // 如果是标准Claude模型名，需要映射为Bedrock格式
     const bedrockModel = this._mapToBedrockModel(selectedModel)
-    if (bedrockModel !== selectedModel) {
+    if (bedrockModel !== selectedModel.replace(/\[1m\]$/, '')) {
       logger.info(`🔄 模型映射: ${selectedModel} → ${bedrockModel}`, {
-        metadata: { originalModel: selectedModel, bedrockModel }
+        metadata: { originalModel: selectedModel, bedrockModel, isLongContext }
       })
     }
 
-    return bedrockModel
+    return { modelId: bedrockModel, isLongContext }
   }
 
   // 将标准Claude模型名映射为Bedrock格式
@@ -546,13 +550,23 @@ class BedrockRelayService {
   }
 
   // 转换Claude格式请求到Bedrock格式
-  _convertToBedrockFormat(requestBody) {
+  _convertToBedrockFormat(requestBody, { isLongContext = false } = {}) {
     const bedrockPayload = {
       anthropic_version: 'bedrock-2023-05-31',
       max_tokens: Math.min(requestBody.max_tokens || this.maxOutputTokens, this.maxOutputTokens),
       messages: requestBody.messages || []
     }
 
+    // Enable 1M context window beta when [1m] model variant is requested
+    if (isLongContext) {
+      bedrockPayload.anthropic_beta = [
+        ...(requestBody.anthropic_beta || []),
+        'context-1m-2025-08-07'
+      ]
+    } else if (requestBody.anthropic_beta) {
+      bedrockPayload.anthropic_beta = requestBody.anthropic_beta
+    }
+
     // 添加系统提示词
     if (requestBody.system) {
       bedrockPayload.system = requestBody.system