fix(examples): use raw DashScope model name

[huangxian119]

Summary

• Fix RuntimeContextExample to pass the raw DashScope API model name qwen-plus when constructing DashScopeChatModel directly.
• Keep the provider-prefixed dashscope:qwen-plus form for ModelRegistry-based configuration.
• Add a regression test covering the direct builder configuration.

Root cause

dashscope:qwen-plus is a ModelRegistry identifier, not a DashScope API model name. Sending it directly to DashScope returns HTTP 400 with InvalidParameter: Model not exist.

Test plan

• JAVA_HOME=<JDK 17> mvn -pl agentscope-examples/documentation -am -Dtest=RuntimeContextExampleTest -DfailIfNoTests=false test
• JAVA_HOME=<JDK 17> mvn -pl agentscope-examples/documentation -am -DskipTests compile

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[codecov]

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✅ All modified and coverable lines are covered by tests.

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