Enable nullability in shared specification tests

[AndriySvyryd]

Part 4 of 5 for #24427. Based on #38790.

Summary

• Enables nullable analysis throughout shared model, query, update, and test utility infrastructure.
• Enables NRT in InMemory functional and ASP.NET specification/functional projects.
• Annotates optional CLR properties and navigations so provider behavior remains unchanged.
• Keeps all pre-existing assertion invocations semantically intact.

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 (this PR)
5. Enable nullability in shared test sources #38792

Validation

• Full EFCore.slnx build at this tip: 0 warnings, 0 errors.
• InMemory functional tests: 28,258 total, 0 failed.
• Final Roslyn assertion audit: 1,503 changed C# files, 0 missing assertions, 0 parse errors.

• 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

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