fix(transport): add HTTP/2 to HTTP/1.1 automatic fallback for connect…

[Mrjyw]

When HTTP/2 connections are dropped by cloud infrastructure
(e.g. Alibaba Cloud SLB GOAWAY, RST_STREAM due to rate limiting),the transport now automatically retries the request with HTTP/1.1
instead of failing immediately. Previously, such connection-level
errors propagated directly to the caller with no protocol fallback.

Fixes #1287

1.0.12-SNAPSHOT

Background: Alibaba Cloud SLB/ALB enforces a default limit of concurrent
streams per HTTP/2 connection. When this limit is exceeded, the load balancer sends
GOAWAY or RST_STREAM frames, causing the JDK HttpClient to throw IOExceptions without
an HTTP status code. Other cloud providers (Tencent Cloud CLB, Huawei Cloud ELB) and
enterprise proxies exhibit similar behavior. JDK HttpClient's built-in ALPN
negotiation cannot handle these mid-connection failures since the HTTP/2 connection
was already successfully negotiated.

Changes:

• JdkHttpTransport: Added http1_1Client field, built eagerly in all three
  constructors via buildClient(config, HttpVersion.HTTP_1_1).

• execute(): Catches HttpTransportException with no status code when HTTP/2 is
  configured, then retries via doSend(http1_1Client, jdkRequest). If the fallback
  also fails, the original error is thrown to preserve the root cause.

• stream(): Added onErrorResume in the reactive chain that catches connection-
  level errors (IOException or HttpTransportException without status code) and
  retries with the HTTP/1.1 client via sendAsyncStream().

• Extracted doSend() and sendAsyncStream() helpers that accept a HttpClient
  parameter, making both protocol paths share the same execution logic.

• buildClient() overloaded to accept an explicit HttpVersion parameter.

• Added 7 tests in JdkHttpTransportTest covering the full decision tree:
  HTTP/2 fallback success (execute + stream), HTTP/1.1 normal success (execute +
  stream), HTTP/1.1 no fallback, HTTP/2 no fallback for HTTP errors, and fallback
  failure propagating original error.

• 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

• Code is ready for review

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ntscope/core/model/transport/JdkHttpTransport.java	83.33%	1 Missing and 6 partials ⚠️

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