PG security enhancement

[xiangyan99]

What does this PR do?

[Provide a clear, concise description of the changes]

1. Disallow keywords: UNION/INTERSECT/EXCEPT
2. Disable more dangerous identifiers

[Any additional context, screenshots, or information that helps reviewers]

GitHub issue number?

[Link to the GitHub issue this PR addresses]

Pre-merge Checklist

• Required for All PRs
  • Read contribution guidelines
  • PR title clearly describes the change
  • Commit history is clean with descriptive messages (cleanup guide)
  • 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)
• For MCP tool changes:
  • One tool per PR: This PR adds or modifies only one MCP tool for faster review cycles
  • Updated servers/Azure.Mcp.Server/README.md and/or servers/Fabric.Mcp.Server/README.md documentation
  • Validate README.md changes using script at eng/scripts/Process-PackageReadMe.ps1. See Package README
  • Updated command list in /servers/Azure.Mcp.Server/docs/azmcp-commands.md and/or /docs/fabric-commands.md
  • Run .\eng\scripts\Update-AzCommandsMetadata.ps1 to update tool metadata in azmcp-commands.md (required for CI)
  • For new or modified tool descriptions, ran ToolDescriptionEvaluator and obtained a score of 0.4 or more and a top 3 ranking for all related test prompts
  • For tools with new names, including new tools or renamed tools, update consolidated-tools.json
  • For renamed tools, follow the Tool Rename Checklist and tag the PR with the breaking-change label
  • For new tools associated with Azure services or publicly available tools/APIs/products, add URL to documentation in the PR description
• Extra steps for Azure MCP Server tool changes:
  • Updated test prompts in /servers/Azure.Mcp.Server/docs/e2eTestPrompts.md
  • 👉 For Community (non-Microsoft team member) PRs:
    • Security review: Reviewed code for security vulnerabilities, malicious code, or suspicious activities before running tests (crypto mining, spam, data exfiltration, etc.)
    • Manual tests run: added comment /azp run mcp - pullrequest - live to run Live Test Pipeline

[Copilot]

Pull request overview

This PR tightens the Postgres SQL query validation used by the Postgres tool to reduce exfiltration/privilege-abuse risk from user-provided SQL, and adds unit tests plus a changelog entry.

Changes:

• Block SQL set-operation keywords (UNION / INTERSECT / EXCEPT) in SqlQueryValidator.
• Expand the blocked identifier list (e.g., pg_user, pg_roles, generate_series, pg_stat_ssl) and adjust string/comment handling.
• Add unit tests for validator behavior and a changelog entry.

Reviewed changes

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

File	Description
tools/Azure.Mcp.Tools.Postgres/src/Validation/SqlQueryValidator.cs	Adds set-operation keyword blocking and updates string/comment scanning in the validator.
tools/Azure.Mcp.Tools.Postgres/tests/Azure.Mcp.Tools.Postgres.UnitTests/Validation/SqlQueryValidatorTests.cs	Introduces new unit tests for allowed/disallowed queries.
servers/Azure.Mcp.Server/changelog-entries/1773853286236.yaml	Records the validator hardening in the server changelog entry feed.