Drop 'unknown' model fallbacks across AI integrations #25353

When a request has no model, the AI integrations set gen_ai.request.model to 'unknown', and the span name becomes <operation> unknown. With span streaming the name drops the model, but the attribute stays.

If there is no model, do not set gen_ai.request.model. The span name is then only the operation (for example chat), with or without span streaming.

Affected integrations (in packages/server-utils/src):

• OpenAI: ai/openai/index.ts, ai/openai/utils.ts, integrations/openai.ts
• Anthropic: ai/anthropic-ai/index.ts
• Google GenAI: ai/google-genai/index.ts, integrations/google-genai.ts
• LangChain: ai/langchain/index.ts, ai/langchain/utils.ts, ai/langchain/embeddings.ts
• Workers AI: ai/workers-ai/index.ts, ai/workers-ai/utils.ts

Raised in review: #25285 (comment)