feat(trigger-sdk): add streamBaseURL to TriggerChatTransport

[ericallam]

TriggerChatTransport had a single baseURL option covering both the .in/append POSTs and the long-lived .out SSE subscription. Customers wanting to route the SSE through a proxy (e.g. a Cloudflare worker capturing JA4 fingerprints for bot detection) had to send every append through the proxy too, adding a hop to every user message.

New optional streamBaseURL overrides the SSE base URL only; appends keep using baseURL. Falls back to baseURL when unset, so existing transports are unchanged.

const transport = new TriggerChatTransport({
  task: "ai-chat",
  baseURL: "https://api.trigger.dev",
  streamBaseURL: "https://chat-proxy.example.com",
  accessToken,
  startSession,
});

Verified with a new test in chat.test.ts that asserts .in/append routes through baseURL and .out SSE routes through streamBaseURL. All existing tests still pass.

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📒 Files selected for processing (2)

• packages/trigger-sdk/src/v3/chat.test.ts
• packages/trigger-sdk/src/v3/chat.ts

🚧 Files skipped from review as they are similar to previous changes (2)

• packages/trigger-sdk/src/v3/chat.ts
• packages/trigger-sdk/src/v3/chat.test.ts

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---

Walkthrough

This PR adds an optional streamBaseURL configuration option to TriggerChatTransport to route long-lived SSE subscriptions to a separate base URL while keeping other session API calls on baseURL. The transport stores streamBaseURL (defaulting to baseURL when unset) and uses it when constructing the /realtime/v1/sessions/{chatId}/out SSE URL. A new test verifies .in/append requests use baseURL and .out subscriptions use streamBaseURL.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: adding a streamBaseURL option to TriggerChatTransport.
Description check	✅ Passed	The description covers the rationale, implementation details, usage example, and testing approach; all key template sections are addressed with substantive content.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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• Commit unit tests in branch feat/chat-transport-stream-base-url

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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