Mock generated-code nullable regression guards leave compiler warnings suppressed #7436

Summary

Mock generator nullable regression helpers can pass even when generated implementations contain nullable warnings. Generated source starts with #pragma warning disable, which compiler WarningsAsErrors cannot override. Restore relevant diagnostics in test compilations and prove enforcement with a deliberately invalid positive control.

Reproduction

Inspect src/TUnit.Mocks.SourceGenerator/MockGenerator.cs (implementation header) and tests/TUnit.Mocks.SourceGenerator.Tests/SnapshotTestBase.cs:221 on main d87516c. AssertGeneratedCodeHasNoNullableWarnings compiles generated trees without restoring the disabled warnings.

Proposal

Re-enable relevant nullable diagnostics in test-only generated trees before compiling and add a positive control proving that an injected nullable mismatch fails the guard. Preserve production generated output and existing reviewed snapshots unless a separate production defect requires change.

This is the nonblocking CodeRabbit follow-up on #7414; call-site CS8634 enforcement remains useful for that fix.

Acceptance criteria

• In test-only generated syntax trees, restore nullable diagnostics after the generated warning suppression; leave production output unchanged.
• Cover the nullable-event regression diagnostics CS8604 and CS8612–CS8615 as well as the existing nullable guards. Project-level WarningsAsErrors alone cannot override generated #pragma warning disable.
• Add a deliberately invalid generated nullable-event signature (or equivalent targeted positive control) and prove the guard fails; verify the valid nullable-event regressions pass.
• Run the affected mock source-generator tests and review any intentional snapshot changes.

The generated-code enforcement requirement from #7432 is consolidated here. PR #7414 removes the unsupported pragmas and adds project-level warning promotion, but does not restore diagnostics disabled inside generated files.