fix(anthropic): tolerate proxy responses without message ids

[Alexxigang]

Summary

• tolerate Anthropic proxy responses that omit the top-level id field
• reuse the SDK raw _id() field when available and fall back gracefully when it is missing
• add parser- and formatter-level regression coverage for both non-streaming and message-start streaming paths

Root cause

AnthropicResponseParser called message.id() directly in both the non-streaming parse path and the streaming message-start path. Some proxy services strip the top-level id field, and the Anthropic SDK throws before AgentScope can build a ChatResponse.

Fix

• add a safe message-id reader that prefers the SDK raw _id() field
• fall back to a generated ChatResponse id when the payload really has no id
• cover raw-id reuse and missing-id fallback in parser and formatter tests

Validation

• mvn -pl :agentscope-core -am spotless:apply
• mvn -pl :agentscope-core -am "-Dtest=AnthropicResponseParserTest,AnthropicChatFormatterTest" test