Additonal index changes to cover postgres

[rbev]

This pull request improves query performance for failure groups on PostgreSQL and refactors per provider entity configurations to use configuration classes.

Entity Configuration Refactoring:

• Moved index definitions for FailedMessageEntity and FailedMessageGroupEntity into dedicated configuration classes (FailedMessageConfiguration and FailedMessageGroupConfiguration) for both PostgreSQL and SQL Server providers, and updated the main DbContext to apply these configurations instead of defining indexes inline. [1] [2] [3] [4] [5]

Index and Query Performance Improvements:

• Enhanced the failed_messages table index to include UniqueMessageId and added covering columns (FirstTimeOfFailure, LastTimeOfFailure) for more efficient queries. The failed_message_groups index now includes both FailedMessageUniqueId and Title for better index-only scans. [1] [2] [3]
• Updated the EF Core model snapshots and migration files to reflect new index definitions and included columns, ensuring the database schema matches the new configuration. [1] [2] [3] [4]

[johnsimons]

There is only a question of lots of (string)null, which I don't get it!

[rbev]

There is only a question of lots of (string)null, which I don't get it!

Yeah they are strange, but they are from generated code.
I can try reverting the last SQLserver migration and see if it happens again, my efcore tooling was recently updated so it could be from that.