Return useful error message for AIs if we fail to deserialize custom datatypes

[jumacabo]

What does this PR do?

Return useful error message for AIs if we fail to deserialize custom data types, the error message contains instructions for getting the table schema, cast them to out-of-the-box data types and retry the operation.

GitHub issue number?

#279

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[Copilot]

Pull Request Overview

This PR adds error handling for unsupported complex PostgreSQL data types by catching InvalidCastException during query result processing. When the Npgsql data reader encounters non-standard data types (such as extension or user-defined types) that cannot be converted to strings, the code now throws a CommandValidationException with detailed guidance for the user on how to resolve the issue.

• Added try-catch block around reader[i]?.ToString() to handle InvalidCastException when reading query results
• Throws CommandValidationException with actionable error message including column name and remediation steps
• Added necessary imports: System.Net and Azure.Mcp.Core.Exceptions

[kk-src]

Can you please add test for this with the PR.

[jumacabo]

Can you please add test for this with the PR.

Hi @kk-src , this simple request had more work than what I have imagined, turns out that the PostgresService class was not prepared for mocking (it creates instances of services directly inside, also some of them cannot be mocked because they are sealed classes, etc.) and all of it's corresponding unit tests (in PostgresServiceParameterizedQueryTests) are not doing what they say they are doing, in fact, throwing an exception at the beginning of their corresponding PostgresService methods don't make them fail.

I have refactored the code so we can use mocked services for Entra and Npgsql, also created a FakeDbDataReader to simulate getting data from Postgres and also created a new set of unit tests that do validate the output of ExecuteQueryAsync (see PostgresServiceTests)

If you agree with the changes please approve the PR, in any case when you have a chance would like to talk to you about the pre-existing unit tests and what we should do to fix them.