Enable nullable annotations in precompiled query interceptors

[nurullahturkoglu]

Fixes #37996

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I posted the plan in #37996 (comment) but haven't heard back from the team yet, so the second box is unchecked.

Description

The generated interceptors file starts with // <auto-generated />, so the compiler disables the nullable context for it, but the interceptor signatures keep the nullable annotations of the operators they intercept (e.g. TSource? for FirstOrDefault). This produces CS8669 in the user's project regardless of its Nullable setting.

The generator now emits #nullable enable annotations after the #pragma warning disable EF9100 line. This makes the annotations valid without enabling nullable warnings for the rest of the generated code.

Testing

PrecompiledQueryTestHelpers.Test now fails if the generated files produce CS8669, so every precompiled query test checks it.

• Without the fix, 33 of 155 tests in PrecompiledQuerySqliteTest and AdHocPrecompiledQuerySqliteTest fail, and 43 in the SQL Server precompiled test classes.
• With the fix, the SQLite and SQL Server precompiled query tests pass (SQL Server ran against a local Linux container on macOS), as do EFCore.Design.Tests and the full EFCore.Sqlite.FunctionalTests.

I haven't run the tests on Windows.