fix: separate Anthropic output and context limits

Direct Anthropic connections reused the configured context window as max_tokens, which could impose an unintended output value or send a provider-invalid request. Anthropic connections now expose an independent per-response maximum output setting, defaulting to 32K for the protocol-required field, while OpenAI direct requests remain without an added output token cap and direct tool rounds remain unmetered.

Release note: Anthropic 直连模型现在可单独配置单次最大输出，不再复用上下文窗口；OpenAI 直连请求和工具调用轮次不增加额外配额。

src/main/agent/model-runtime.test.ts

@@ -476,6 +476,7 @@ describe('ModelAgentRuntime', () => {
       model: 'sonnet-5',
       protocol: 'anthropic-messages',
       authentication: 'api-key',
+      maximumOutputTokens: 48_000,
       skillInstructions: '# 文档写作',
       fetcher
     })
@@ -500,11 +501,13 @@ describe('ModelAgentRuntime', () => {
 
     const body = JSON.parse(init?.body as string) as {
       model: string
+      max_tokens: number
       stream: boolean
       system: string
     }
     expect(body).toMatchObject({
       model: 'sonnet-5',
+      max_tokens: 48_000,
       stream: true
     })
     expect(body.system).toMatch(