Fix multiple public constructors for xUnit v3 compatibility

[Copilot]

xUnit v3 (used in .NET 10) enforces single public constructor per test class. Four classes violated this constraint, causing test discovery to fail.

Changes

• TestMySqlRetryingExecutionStrategy: Made 3 unused constructors internal, keeping only ExecutionStrategyDependencies constructor public
• MySqlConditionalFactTestCase/TheoryTestCase: Added [EditorBrowsable(EditorBrowsableState.Never)] to obsolete parameterless constructors (required for xUnit serialization but should be hidden from discovery)
• EntitySplittingQueryMySqlTest: Made ITestOutputHelper convenience constructor internal, keeping primary constructor with fixture public

Example

Before:

public class TestMySqlRetryingExecutionStrategy : MySqlRetryingExecutionStrategy
{
    public TestMySqlRetryingExecutionStrategy() { ... }
    public TestMySqlRetryingExecutionStrategy(DbContext context) { ... }
    public TestMySqlRetryingExecutionStrategy(DbContext context, TimeSpan maxDelay) { ... }
    public TestMySqlRetryingExecutionStrategy(ExecutionStrategyDependencies dependencies) { ... }
}

After:

public class TestMySqlRetryingExecutionStrategy : MySqlRetryingExecutionStrategy
{
    internal TestMySqlRetryingExecutionStrategy() { ... }
    internal TestMySqlRetryingExecutionStrategy(DbContext context) { ... }
    internal TestMySqlRetryingExecutionStrategy(DbContext context, TimeSpan maxDelay) { ... }
    public TestMySqlRetryingExecutionStrategy(ExecutionStrategyDependencies dependencies) { ... }
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fix Error: A test class may only define a single public constructor.</issue_title>
<issue_description>Doing running Functional tests, we have this error, so find and fix all those places where that is the case:

t test/EFCore.MySql.FunctionalTests -c Debug --no-build --logger "GitHubActions;report-warnings=false" --verbosity detailed

Remember that is .net core 10</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Fix Error: A test class may only define a single public constructor. #110

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