Prevent Dock JSON source-generation context collisions

[wieslawsoltes]

Summary

• restrict Dock's nested System.Text.Json generator output to DockSystemTextJsonContext.*
• leave application-owned JsonSerializerContext artifacts to the SDK generator
• add an integration regression with two nested application contexts matching the reported scenario

Root cause

Dock runs the System.Text.Json generator internally to materialize metadata for its generated context. That nested run sees every JsonSerializerContext in the consumer compilation, and Dock previously republished every resulting artifact. The SDK generator also publishes the application contexts during the normal build, so their partial members and generated attributes were emitted twice.

Impact

Consumers can combine [assembly: DockJsonSourceGeneration] with one or more application JsonSerializerContext classes. Dock continues to emit its own metadata context, while application contexts retain their normal SDK-generated APIs.

Generator pipeline

The semantic input remains the complete consumer compilation so Dock types resolve normally. The per-context emission boundary now filters the nested generator result by Dock's stable context hint-name prefix; shared resolver generation is unchanged. This keeps output ownership deterministic without altering discovery or runtime behavior.

Validation

• reproduced the original failure with two nested contexts: 52 duplicate-member/attribute compilation errors before the fix
• dotnet test tests/Dock.Serializer.SystemTextJson.SourceGenTests/Dock.Serializer.SystemTextJson.SourceGenTests.csproj --filter FullyQualifiedName~DockGenerator_CoexistsWithApplicationJsonContexts --no-restore -maxcpucount:1 (1 passed)
• dotnet test tests/Dock.Serializer.SystemTextJson.Generators.UnitTests/Dock.Serializer.SystemTextJson.Generators.UnitTests.csproj --no-restore -maxcpucount:1 (8 passed)
• dotnet test tests/Dock.Serializer.SystemTextJson.SourceGenTests/Dock.Serializer.SystemTextJson.SourceGenTests.csproj --no-build --no-restore -maxcpucount:1 (13 passed)
• dotnet build samples/DockSystemTextJsonSourceGenAotSample/DockSystemTextJsonSourceGenAotSample.csproj --no-restore -maxcpucount:1 (succeeded)
• generated-file audit: application contexts appear only under System.Text.Json.SourceGeneration; Dock output contains only DockSystemTextJsonContext.*
• git diff --check

Fixes #1085

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