Fix #6953 projection cast on explicit union root fields

[michaelstaib]

Fixes #6953

Summary

• add a regression test for [UseProjection] on a root field returning IQueryable<Base> while exposing ListType<UnionType>
• reproduce both reported query shapes (__typename only and fragment selections)
• fix QueryableProjectionProvider to use TEntityType when the unwrapped selection runtime type is object, so the generated projection lambda keeps the correct source parameter type

Test Commands

• dotnet test src/HotChocolate/Data/test/Data.Tests/HotChocolate.Data.Tests.csproj --filter "FullyQualifiedName~Issue6953Tests|FullyQualifiedName~Issue5528ReproTests" --nologo

Results

• Passed on net8.0/net9.0/net10.0 (Failed: 0)

[github-actions]

Fusion Gateway Performance Results

Simple Composite Query

	Req/s	Err%
Constant 2 (50 VUs)	3214.99	0.00%
Ramping 2 (0-500-0 VUs)	3936.78	0.00%

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	1.11ms	12.51ms	15.26ms	24.78ms	34.11ms	259.29ms
Ramping 2	1.02ms	50.32ms	55.74ms	97.13ms	137.08ms	280.02ms

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Deep Recursion Query

	Req/s	Err%
Constant 2 (50 VUs)	765.78	0.00%
Ramping 2 (0-500-0 VUs)	926.82	0.00%

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	9.00ms	55.22ms	63.24ms	81.44ms	102.78ms	705.85ms
Ramping 2	2.89ms	199.84ms	222.79ms	457.33ms	515.91ms	971.77ms

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Variable Batching Throughput

	Req/s	Err%
Constant 2 (50 VUs)	5603.22	0.00%
Ramping 2 (0-500-0 VUs)	5275.31	0.00%

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	0.15ms	8.31ms	8.82ms	14.76ms	17.93ms	56.75ms
Ramping 2	0.19ms	38.90ms	43.54ms	81.57ms	106.15ms	220.44ms

---

Runner 2 = benchmarking-2

Run 22496070137 • Commit d597e04 • Fri, 27 Feb 2026 17:18:25 GMT