Make cursor key extraction null-safe for nullable nested fields

[glen-84]

Summary

• Sorting a cursor-paginated connection on a nullable nested field (e.g. order: [{ meter: { name: ASC } }]) crashed with "Unexpected Execution Error" whenever an intermediate navigation was null: the cursor key selector was compiled and invoked verbatim, so it dereferenced the null and threw. The crash surfaced for Connection-returning resolvers (which eagerly create start/end cursors) even when no cursor field was selected.
• CursorKey now compiles the selector into a null-safe delegate (NullSafeKeySelector) that mirrors the C# null-conditional operator: each nullable intermediate is bound to a local and evaluated once, reference / HasValue null checks short-circuit to a null key value (which the serializer already formats as the null marker), and intermediate casts/conversions are preserved so member access rebinds against the correct type.

Test plan

• Added CursorFormatterTests covering null and non-null nested intermediates, deep chains, single evaluation (counting getter), overloaded ==, a nullable value-type intermediate, and a cast intermediate.
• GreenDonut.Data.Tests: 364/364 pass.
• PostgreSQL nullable cursor round-trip tests: 9/9 pass.

[Copilot]

Pull request overview

This PR fixes cursor key extraction for cursor-paginated connections when sorting by nullable nested fields by compiling cursor key selectors into a null-safe delegate that short-circuits to a null key instead of throwing on null intermediates.

Changes:

• Compile cursor key selector expressions into a null-safe delegate (NullSafeKeySelector) that mirrors C#’s null-conditional behavior and evaluates intermediates once.
• Add unit tests covering null/non-null nested intermediates, deep chains, single evaluation, overloaded equality, nullable value-type intermediates, and cast intermediates.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/GreenDonut/src/GreenDonut.Data/Cursors/CursorKey.cs	Switches key selector compilation to a null-safe compiler to prevent null dereference crashes during cursor formatting.
src/GreenDonut/test/GreenDonut.Data.Tests/Cursors/CursorFormatterTests.cs	Adds focused tests validating null-safe nested cursor key formatting behavior across several edge cases.

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[github-code-quality]

Code Coverage Overview

Languages: C#

C# / code-coverage/dotnet

The overall coverage in the gai/null-safe-nested... branch remains at 49%, unchanged from the main branch.

Show a code coverage summary of the most impacted files.

File	main 03ba3ad	gai/null-safe-nested... 609c3a5	+/-
/home/runner/wo...ageProcessor.cs	100%	89%	-11%
/home/runner/wo...heMiddleware.cs	92%	86%	-6%
/home/runner/wo...boxProcessor.cs	72%	74%	+2%
/home/runner/wo...DefaultTopic.cs	88%	90%	+2%
/home/runner/wo...tocolHandler.cs	82%	84%	+2%
/home/runner/wo...efaultPubSub.cs	89%	91%	+2%
/home/runner/wo...geDispatcher.cs	73%	77%	+4%
/home/runner/wo...r.Observable.cs	78%	85%	+7%
/home/runner/wo...erMiddleware.cs	89%	97%	+8%
/home/runner/wo...gerMessage.g.cs	11%	19%	+8%

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_{Updated June 19, 2026 11:20 UTC
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