Refactor sub fallback

[xiangyan99]

What does this PR do?

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Refactor the subscription fallback logic. Attempt env var only when the provided one does not exist.

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Pre-merge Checklist

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  • [ ] Added comprehensive tests for new/modified functionality
  • [ ] Updated servers/Azure.Mcp.Server/CHANGELOG.md and/or servers/Fabric.Mcp.Server/CHANGELOG.md for product changes (features, bug fixes, UI/UX, updated dependencies)
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  • [ ] Updated command list in /servers/Azure.Mcp.Server/docs/azmcp-commands.md and/or /docs/fabric-commands.md
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• [ ] Extra steps for Azure MCP Server tool changes:
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    • [ ] Manual tests run: added comment /azp run mcp - pullrequest - live to run Live Test Pipeline