fix(transport): resolve SSE stream timeout in JdkHttpTransport

[jujn]

Description

Close #1302
This PR introduces a structural fix for JdkHttpTransport when handling long-running Server-Sent Events (SSE) streams, particularly for LLMs with long Time-To-First-Token (TTFT) requirements.
Previously, the combination of JDK HttpClient's absolute timeout semantics and synchronous stream reading within a reactive pipeline led to premature request terminations, leaked connections, and severe NIO thread pool starvation.

Key Changes & Architectural Improvements:

• Removed Absolute Timeout for Streams: Bypassed the java.net.http.HttpRequest.timeout() for streaming requests. The JDK's absolute timeout forcefully cuts off connections regardless of ongoing data transfer, which is fundamentally incompatible with LLM streaming.

• Dual-Stage Reactive Timeout Strategy:
  Moved timeout management to the Reactor layer using a custom timeout() operator:

  • responseTimeout (TTFT): Tolerates long-thinking models waiting for the first byte/token (Default: 5 mins).
  • streamIdleTimeout (Inter-token gap): Resets upon receiving every data chunk (Default: 30 seconds).

• Prevent NIO Thread Starvation:
  Added .subscribeOn(Schedulers.boundedElastic()) after the HTTP response is received. This guarantees that the blocking I/O operations (BufferedReader.readLine()) used to parse the NDJSON/SSE stream do not hijack and block the JDK HttpClient's highly limited internal NIO selector threads.

• Fixed "Ghost Connection" Leaks:
  Wrapped the asynchronous JDK request in Mono.fromFuture(() -> client.sendAsync(...)). The Supplier ensures lazy execution, and using fromFuture (instead of fromCompletionStage) guarantees that Reactor's cancellation signals correctly trigger future.cancel(true), closing the underlying socket if the downstream aborts or times out before headers arrive.

Checklist

Please check the following items before code is ready to be reviewed.

• Code has been formatted with mvn spotless:apply
• All tests are passing (mvn test)
• Javadoc comments are complete and follow project conventions
• Related documentation has been updated (e.g. links, examples, etc.)
• Code is ready for review

[codecov]

Codecov Report

❌ Patch coverage is 84.44444% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ntscope/core/model/transport/JdkHttpTransport.java	78.78%	3 Missing and 4 partials ⚠️

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