feat(azure): forward credential_scopes to Azure AI Inference client (#5661)

* feat(azure): forward credential_scopes to Azure AI Inference client

Adds a credential_scopes field to the native Azure AI Inference
provider and a matching AZURE_CREDENTIAL_SCOPES env var
(comma-separated). The value is forwarded to ChatCompletionsClient /
AsyncChatCompletionsClient when set, letting keyless / Entra-based
callers target a specific Azure AD audience (e.g.
https://cognitiveservices.azure.com/.default) without subclassing the
provider. Matches the upstream azure.ai.inference SDK kwarg of the
same name.

Lazy build re-reads the env var so an LLM constructed at module
import (before deployment env vars are set) still picks up scopes —
same pattern as the existing AZURE_API_KEY / AZURE_ENDPOINT lazy
reads. to_config_dict round-trips the field.

* refactor(azure): tighten credential_scopes env handling

Address review feedback:
- Move os.getenv into the helper so AZURE_CREDENTIAL_SCOPES appears once
- Match the surrounding api_key/endpoint `or` style in the validator
- Drop the list() defensive copy in to_config_dict — every other field
  in that method (and the base class's `stop`) is assigned by reference