Use ConnectionStringBuilder instead of string interpolation

[xiangyan99]

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Use ConnectionStringBuilder instead of string interpolation

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Pre-merge Checklist

• Required for All PRs
  • Read contribution guidelines
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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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[Copilot]

Pull request overview

This PR hardens the PostgreSQL and MySQL tools against connection-string injection by switching from string interpolation to provider connection-string builders, and adds regression tests plus a server changelog entry documenting the fix.

Changes:

• PostgreSQL: replace interpolated connection strings with NpgsqlConnectionStringBuilder via a new helper.
• MySQL: replace interpolated connection strings with MySqlConnectionStringBuilder via a new helper (and expose it for unit testing).
• Add regression unit tests for connection-string injection scenarios and a changelog entry.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
tools/Azure.Mcp.Tools.Postgres/tests/Azure.Mcp.Tools.Postgres.UnitTests/Services/PostgresServiceConnectionStringInjectionTests.cs	New regression tests validating injected database values don’t override Host / SSL settings.
tools/Azure.Mcp.Tools.Postgres/src/Services/PostgresService.cs	Uses a new BuildConnectionString helper to build safe PostgreSQL connection strings.
tools/Azure.Mcp.Tools.MySql/tests/Azure.Mcp.Tools.MySql.UnitTests/Services/MySqlServiceConnectionStringInjectionTests.cs	New regression tests validating injected database/user values don’t override Server / SSL settings.
tools/Azure.Mcp.Tools.MySql/src/Services/MySqlService.cs	Uses MySqlConnectionStringBuilder via a new internal BuildConnectionString helper (SSL required).
servers/Azure.Mcp.Server/changelog-entries/1773675545058.yaml	Records the security bug fix in the server changelog.