fix(strawberryshake): dedupe duplicate fragment spreads in interface list (CS0528)

[cliedeman]

Problem

Spreading the same named fragment two or more times in one selection set makes Strawberry Shake emit the fragment's generated interface twice in the operation result type's base-interface list. Code generation reports no error, but the generated C# fails to compile:

error CS0528: 'IUF' is already listed in interface list

Minimal reproductions (valid GraphQL, fail to compile before this change)

query Q { me { ...UF ...UF } }            fragment UF on User { id name }
query Q { node(id: "1") { ...NF ...NF } }  fragment NF on Node { id }

These generated, respectively:

public partial interface IQ_Me : IUF, IUF      // CS0528
public partial interface IQ_Node : INF, INF    // CS0528

Root cause

• FieldCollector.ResolveFragmentSpread adds one FragmentNode per spread, so two ...UF spreads produce two nodes for the same fragment.
• FragmentHelper.CreateImplements iterates those nodes and, for each, adds the interface OutputTypeModel returned by CreateInterface. Because models are cached one-per-name, the same instance is added once per spread.
• The existing dedup in CreateInterface (IsImplementedBy) only removes a model that is implemented by another model in the list, not a literal duplicate, so the duplicate base interface flows through to TypeDescriptorMapper.ExtractImplementsBy and into the generated C#.

Fix

In FragmentHelper.CreateImplements, skip an interface model that is already present in the implements list. Models are cached one-per-name, so reference identity is sufficient and the change is minimal.

src/StrawberryShake/CodeGeneration/src/CodeGeneration/Analyzers/FragmentHelper.cs

Output is unchanged for single spreads, two distinct fragments, the same fragment at different selection levels, and union members (verified by control tests and the full existing suite).

Tests

New DuplicateFragmentSpreadGeneratorTests covers both repros plus four control/regression guards (single spread, two different fragments, same fragment at different levels, same fragment twice on a union member). GeneratorTestHelper.AssertResult Roslyn-compiles the generated output, so a CS0528 fails the test.

• Before fix: the two repro tests fail with CS0528 ... already listed in interface list; the four controls pass.
• After fix: all 6 pass; full StrawberryShake.CodeGeneration.CSharp.Tests project green (195 passed, 1 pre-existing defer skip, 0 failed).

— Claude
🤖 Generated with Claude Code

[github-actions]

Patch coverage

100.0% of changed lines covered (6/6)

File	Covered	Changed	Patch %
…/src/CodeGeneration/Analyzers/FragmentHelper.cs	6	6	100.0% 🟢

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Project coverage: 52.4% (215301/411033 lines)