Enable nullability in provider tests

[AndriySvyryd]

Part 2 of 5 for #24427. Based on #38788.

Summary

• Enables nullable analysis across Cosmos, SQLite, SQL Server, OData, cross-store, and HierarchyId tests.
• Corrects optional provider test model members so NRT annotations do not change EF model semantics.
• Preserves every pre-existing SQL Server and SQLite AssertSql raw string.
• Includes the nullable query sorter contract required by provider overrides.

Stack

1. Enable nullability in benchmarks and core tests #38788
2. Enable nullability in provider tests #38789 (this PR)
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

Validation

• Full EFCore.slnx build at this tip: 0 warnings, 0 errors.
• Encoding-safe SQL baseline audit at the final tip: 136 files with pre-existing baselines, 0 modified or removed baselines.
• Final functional runs: SQLite 38,414 tests and SQL Server 50,812 tests, both with 0 failures; Cosmos also passes with 0 failures.

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• 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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