EF-285: Map discriminator to "_t" unless it has been explicitly configured

[ajcvickers]

This reverts any clobbering by naming conventions.

[Copilot]

Pull request overview

This PR fixes an issue where naming conventions (like CamelCase) were overriding the default discriminator element name. It introduces a new MongoDiscriminatorNamingConvention that runs during model finalization to ensure discriminators are mapped to "_t" unless explicitly configured otherwise.

Changes:

• Added MongoDiscriminatorNamingConvention to preserve "_t" as the discriminator element name
• Registered the new convention in MongoConventionSetBuilder
• Refactored test infrastructure to use generic CamelCaseDbContext<T> for easier test creation
• Added three test cases to verify discriminator naming behavior with CamelCase convention

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
src/MongoDB.EntityFrameworkCore/Metadata/Conventions/MongoDiscriminatorNamingConvention.cs	New convention that ensures discriminators use "_t" unless explicitly configured
src/MongoDB.EntityFrameworkCore/Metadata/Conventions/MongoConventionSetBuilder.cs	Registers the new discriminator naming convention
tests/MongoDB.EntityFrameworkCore.FunctionalTests/Metadata/Conventions/CamelCaseElementNameConventionTests.cs	Refactored test context and added three test cases for discriminator preservation

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[damieng]

Let's update BREAKING-CHANGES.md with a new section for the next release with notes about this and the potential for throwing now the new name is _t if they were previously using CamelCaseNamingConvention.

[ajcvickers]

Updated breaking changes doc.