fix: increase verification max_tokens to 1024 for Poe API compatibility

Poe API's Extended Thinking models (e.g. claude-sonnet-4.6) require
budget_tokens >= 1024. The previous values (5 for OpenAI, 16 for
Anthropic) caused HTTP 400 errors during provider verification.

Fixes #23433

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/commands/onboard-custom.ts

@@ -303,7 +303,7 @@ async function requestOpenAiVerification(params: {
     body: {
       model: params.modelId,
       messages: [{ role: "user", content: "Hi" }],
-      max_tokens: 5,
+      max_tokens: 1024,
     },
   });
 }
@@ -329,7 +329,7 @@ async function requestAnthropicVerification(params: {
     headers: buildAnthropicHeaders(params.apiKey),
     body: {
       model: params.modelId,
-      max_tokens: 16,
+      max_tokens: 1024,
       messages: [{ role: "user", content: "Hi" }],
     },
   });