fix(openai): handle null statusCode in HttpTransportException to prevent NPE

[shlokmestry]

1.0.10-SNAPSHOT

Fixes a NullPointerException in OpenAIClient.stream() caused by auto-unboxing
a nullable Integer when HttpTransportException has no HTTP status code
(e.g. connection failure, DNS error, timeout).

The existing code passed getStatusCode() (return type Integer, nullable)
directly to OpenAIException.create(int, ...), which throws an NPE when
statusCode is null.

Fix: Added a null check before accessing getStatusCode(). If null, falls back
to new OpenAIException(msg, ex) instead of crashing.

Fixes #891

[shlokmestry]

Ran OpenAIClientExceptionTest locally : 17/17 tests pass (Failures: 0, Errors: 0). The only failing test in the full suite (ShellCommandToolTest$BufferDeadlockTests) is unrelated to this change. CI failures are due to Codecov token not being configured for fork PRs and a Windows build timeout, not the code change itself.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...in/java/io/agentscope/core/model/OpenAIClient.java	50.00%	2 Missing and 2 partials ⚠️

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