Handle Azure OpenAI model prefixes as deployments

[Haihan-Jiang]

Summary

• normalize Azure OpenAI deployment names when the model is passed as azure/<deployment> or azopenai/<deployment>
• use the normalized deployment name for chat and completion requests
• add unit coverage for plain and prefixed deployment names

Why

With the azopenai provider, a model value such as azure/gpt-4.1 was passed directly to the Azure SDK as the deployment name. That makes the request path include /deployments/azure/gpt-4.1/..., which Azure reports as DeploymentNotFound.

Fixes #604.

Tests

• go test -run '^TestAzureOpenAIDeploymentName$' -count=1 -timeout=30s .\n- go test -run '^$' -count=1 -timeout=30s .\n- git diff --check\n