Add default Azure credential fallback for Analysis Services

[coleheflin]

Add default Azure identity authentication to the Analysis Services hook when both connection login and password are empty. This is the small follow-up suggested in #65879 (review) after the original feature was merged in #71350.

Use get_async_default_azure_credential because the merged hook is asynchronous. Preserve explicit client-secret authentication and validation of incomplete credentials. Expose the standard managed/workload identity connection fields and document the authentication choices and authorization requirements.

Add tests for credential caching, token scope, asynchronous cleanup, identity extras allowlisting, and client-secret precedence.

Validation:

• Ruff formatting/lint and non-Docker pre-commit checks passed.
• uvx --from uv==0.11.29 uv lock completed with no lockfile changes.
• Attempted the hook unit tests, provider YAML validation, and manual provider mypy check; all are blocked locally because Docker is not installed. The Docker-dependent provider YAML hook was skipped when committing.
• Ran breeze verify; its recommended tests and documentation builds also require Docker and remain unverified. Keep this PR in draft until CI validation completes.

Related: #65879, #71350.

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Was generative AI tooling used to co-author this PR?

• Yes — Pi (gpt-6-astra)

Generated-by: Pi (gpt-6-astra) following the guidelines