fix(model): parse non-standard rate limit errors in OpenAIClient to enable auto-retries

[jujn]

Description

Close #761
Fixes an issue where non-standard gateway errors were swallowed in OpenAIClient, ensuring rate limit (429) errors correctly trigger the auto-retry mechanism.

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

[Copilot]

Pull request overview

This PR addresses #761 by ensuring OpenAI-compatible “gateway” responses that embed rate-limit errors in a non-standard shape (e.g., {code, message, status}) are detected and surfaced as proper OpenAI exceptions so the auto-retry logic can trigger.

Changes:

• Extend OpenAIResponse to capture non-standard error fields (code, message, status) and treat them as errors via isError().
• Update OpenAIClient to prefer outer-layer code/message when standard error is absent, and to map "429" body codes to HTTP 429 for retry classification.
• Add/extend unit tests to cover non-standard error detection in both sync and streaming flows.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
agentscope-core/src/main/java/io/agentscope/core/model/OpenAIClient.java	Improves error extraction and attempts to classify non-standard 429 bodies as rate-limit exceptions.
agentscope-core/src/main/java/io/agentscope/core/formatter/openai/dto/OpenAIResponse.java	Adds non-standard error fields and broadens isError() detection.
agentscope-core/src/test/java/io/agentscope/core/model/OpenAIClientTest.java	Adds regression tests for non-standard 429 errors in sync and streaming.
agentscope-core/src/test/java/io/agentscope/core/formatter/openai/dto/OpenAIResponseTest.java	Adds focused tests for standard vs non-standard error detection.

[codecov]

Codecov Report

❌ Patch coverage is 62.16216% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...cope/core/formatter/openai/dto/OpenAIResponse.java	61.11%	3 Missing and 4 partials ⚠️
...in/java/io/agentscope/core/model/OpenAIClient.java	63.15%	5 Missing and 2 partials ⚠️

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