Allow KQL comments and let statements

[xiangyan99]

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#2383

Pre-merge Checklist

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[Copilot]

Pull request overview

This PR updates the Kusto KQL query validator to permit common, read-only KQL constructs (line comments and let/semicolon usage) that were previously rejected, aligning tool behavior with real-world KQL usage while still blocking tautology injections and management commands.

Changes:

• Remove the rule that blocks // comments in KQL queries.
• Remove the rule that blocks semicolons / multiple statements (enabling let ...; patterns).
• Add/update unit tests and add a changelog entry documenting the behavior change.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
tools/Azure.Mcp.Tools.Kusto/src/Validation/KqlQueryValidator.cs	Removes comment and semicolon rejection logic, leaving tautology + management-command checks in place.
tools/Azure.Mcp.Tools.Kusto/tests/Azure.Mcp.Tools.Kusto.UnitTests/KqlQueryValidatorTests.cs	Updates tests to expect comments/let/semicolons to be allowed; refines management-command test coverage.
servers/Azure.Mcp.Server/changelog-entries/1775838003897.yaml	Documents the validator behavior change as a bug fix.