Split merge tests and code cleanup

[PhenX]

No description provided.

[Copilot]

Pull Request Overview

This PR splits existing merge tests into provider-specific files and performs cleanup in the basic tests base class.

• Adds separate MergeTests classes for Sqlite, SqlServer, PostgreSql, and MySql, each inheriting from the shared base.
• Cleans up BasicTestsBase.cs: removes unused using, simplifies exclusion lambdas, deletes commented-out duplicate tests, and adjusts the test workload size.
• Updates the many-entity insert test count and refactors some lambda parameter names.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/.../Merge/MergeTestsSqlite.cs	New Sqlite-specific merge tests subclass
tests/.../Merge/MergeTestsSqlServer.cs	New SqlServer-specific merge tests subclass
tests/.../Merge/MergeTestsPostgreSql.cs	New PostgreSql-specific merge tests subclass
tests/.../Merge/MergeTestsMySql.cs	New MySql-specific merge tests subclass
tests/.../Merge/MergeTestsBase.cs	Introduces shared merge tests logic in a generic base
tests/.../Basic/BasicTestsBase.cs	Removes unused import, simplifies lambdas, drops duplicated tests, changes test count

Comments suppressed due to low confidence (1)

tests/PhenX.EntityFrameworkCore.BulkInsert.Tests/Tests/Basic/BasicTestsBase.cs:137

• [nitpick] The test count was reduced from 156055 to 56055 without any explanation. Please add a comment clarifying why the workload size was changed or update the test name to reflect the new scale.

const int count = 56055;