Upgrade Hot Chocolate to 16.6.4

[Mohit G (gmohit21)]

Summary

Upgrade all six centrally managed Hot Chocolate packages from 16.0.0 to 16.6.4.

Because this crosses minor versions, the upgrade also requires two compatibility adjustments: handling relational create-input defaults correctly and replacing an obsolete fragment-spread constructor.

File-by-file changes

File	What changed and why
src\Directory.Packages.props	Updates all six direct Hot Chocolate package versions to 16.6.4, keeping the package family aligned.
src\Service.GraphQLBuilder\Mutations\CreateMutationBuilder.cs	Passes the database type into scalar-input generation and stops copying relational database default expressions into GraphQL input defaults. Expressions such as ((1)) and (getdate()) can fail the upgraded library's input-default validation. Defaulted SQL fields remain optional so the database evaluates omitted values; Cosmos literal defaults are preserved.
src\Service.Tests\GraphQLBuilder\MutationBuilderTests.cs	Adds executable-schema regression coverage for SQL defaults across database types, omitted fields, explicit nulls, supplied values, required fields, and single/bulk linking inputs. Uses DAB's actual parameter-extraction path to verify field-presence behavior. Strengthens existing Cosmos default tests to verify that omitted fields still receive their literal defaults.
src\Service.Tests\UnitTests\SqlQueryStructureHelperTests.cs	Replaces the obsolete three-argument FragmentSpreadNode constructor with the overload accepting an explicit empty arguments collection. This avoids an obsolete-API warning becoming a build error while preserving the original test behavior.
docs\design\HC16-upgrade.md	Adds a 16.6.4 follow-up section explaining the compatibility changes and default-value semantics, while retaining the historical 16.0.0 migration guidance.

Behavior preserved

• Relational fields with database defaults remain optional in create inputs.
• Omitting a field remains distinct from explicitly supplying null.
• Database constraints continue to govern explicitly supplied nulls.
• Non-null fields without defaults remain required.
• Cosmos literal defaults and stored-procedure parameter defaults are unchanged

[Copilot]

🟢 Approval recommended

The upgrade-related API adjustments and default-value semantics are implemented consistently and are backed by expanded regression coverage for the affected schema/input behaviors.

Pull request overview

This PR upgrades the repository’s centrally managed Hot Chocolate dependencies from 16.0.0 to 16.6.4 and applies the necessary compatibility adjustments so GraphQL schema generation and input-default semantics remain consistent across relational providers and Cosmos.

Changes:

• Bumps the six centrally managed Hot Chocolate packages to 16.6.4.
• Updates create-input generation to avoid treating SQL default expressions as GraphQL input defaults while preserving Cosmos literal defaults.
• Adds/strengthens regression tests around create-input defaults (omission vs explicit null, required fields, and linking/multi-create), and updates a unit test to use the non-obsolete FragmentSpreadNode constructor.

File summaries

File	Description
src/Directory.Packages.props	Updates centrally managed Hot Chocolate package versions to 16.6.4.
src/Service.GraphQLBuilder/Mutations/CreateMutationBuilder.cs	Passes DatabaseType into scalar-input generation and suppresses GraphQL defaults for relational DB default expressions.
src/Service.Tests/GraphQLBuilder/MutationBuilderTests.cs	Adds executable-schema regression tests for SQL default-expression handling and strengthens Cosmos default-value behavior assertions.
src/Service.Tests/UnitTests/SqlQueryStructureHelperTests.cs	Replaces obsolete FragmentSpreadNode constructor usage with the newer overload including empty arguments.
docs/design/HC16-upgrade.md	Documents the 16.6.4 follow-up and the default-value compatibility rationale/semantics.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0
• Review effort level: Lite

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