Fixed snapshot lookup to use exact match

[xiangyan99]

What does this PR do?

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Fixed snapshot lookup to use exact match

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Pre-merge Checklist

• Required for All PRs
  • Read contribution guidelines
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  • Updated servers/Azure.Mcp.Server/README.md and/or servers/Fabric.Mcp.Server/README.md documentation
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[Copilot]

Pull request overview

This PR fixes Azure File Shares snapshot lookup behavior to avoid ambiguous substring matching when locating snapshots by resource ID.

Changes:

• Updated snapshot lookup logic to require an exact (case-insensitive) match on snapshot resource ID rather than using substring matching.
• Added a server changelog entry documenting the bug fix.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tools/Azure.Mcp.Tools.FileShares/src/Services/FileSharesService.cs	Changes snapshot selection logic to use exact resource ID match instead of Contains, preventing false-positive matches.
servers/Azure.Mcp.Server/changelog-entries/1774298605674.yaml	Adds changelog entry for the snapshot lookup bug fix.