Enable nullability in shared test sources

[AndriySvyryd]

Part 5 of 5 for #24427. Based on #38791.

Summary

• Removes nullable opt-out directives from the 18 shared source files linked into test projects.
• Completes nullable analysis coverage across benchmark and test projects.
• Retains nullable directives only in generated files and tests that intentionally exercise a specific nullable context.

Stack

1. Enable nullability in benchmarks and core tests #38788
2. Enable nullability in provider tests #38789
3. Enable nullability in relational and design tests #38790
4. Enable nullability in shared specification tests #38791
5. Enable nullability in shared test sources #38792 (this PR)

Validation

• Every stack tip builds EFCore.slnx with 0 warnings and 0 errors.
• All 28 runnable test hosts pass serially: 140,057 total, 0 failed, 138,733 passed, 1,324 skipped.
• Trimming apps publish and both return the required success code 100.
• NativeAOT's NRT-specific generated-code error is fixed; remaining NativeAOT failures are stale compiled-model API errors already present on main, and that project is excluded from CI.
• Project nullable opt-outs: 0.
• Retained nullable directives: 424, all generated or targeted.
• Final tree is byte-for-byte identical to the validated snapshot.

Fixes #24427

• I've read the guidelines for contributing and seen the walkthrough
• I've posted a comment on an issue with a detailed description of how I am planning to contribute and got approval from a member of the team
• The code builds and tests pass locally (also verified by our automated build checks)
• Commit messages follow the required format
• Tests for the changes have been added or existing coverage validates the migration
• Code follows the same patterns and style as existing code in this repo

[Copilot]

Pull request overview

This PR removes file-level #nullable enable directives from shared source files so they inherit the nullable context from the consuming projects, completing nullable analysis coverage across test/benchmark builds as part of #24427.

Changes:

• Removed top-of-file #nullable enable directives from shared utility sources linked into multiple projects.
• Standardized nullable context handling by relying on project-level nullable configuration instead of per-file directives.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Shared/SharedTypeExtensions.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/PropertyInfoExtensions.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/OrderedDictionary.ValueCollection.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/OrderedDictionary.KeyCollection.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/OrderedDictionary.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/NonCapturingLazyInitializer.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/Multigraph.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/MemberInfoExtensions.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/IDictionaryDebugView.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/HashHelpers.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/ExpressionVisitorExtensions.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/ExpressionExtensions.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/EnumerableExtensions.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/DisposableExtensions.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/DictionaryExtensions.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/CodeAnnotations.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/Check.cs	Drops file-level nullable directive to rely on project nullable settings.
src/Shared/BidirectionalDictionary.cs	Drops file-level nullable directive to rely on project nullable settings.

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