Fix C# ClientResult conversions for streaming events

[JoshLove-msft]

Streaming event models are never returned as plain ClientResult values, so generating explicit conversions for them adds unusable API surface.

Summary

• Stop treating streaming payload models as root ClientResult outputs.
• Preserve conversion operators for models returned by ordinary protocol methods.
• Add SSE and JSON Lines regression coverage and regenerate streaming fixtures.

Testing

• npm run build
• npm run test:generator
• npm run test:emitter
• npm run format
• Targeted oxlint
• npm run cop

Fixes: #11856

[Copilot]

🟢 Approval recommended

The core logic change aligns with the reported bug and is covered by a targeted regression test; remaining feedback is a non-blocking test-assertion precision improvement.

Pull request overview

This PR adjusts the C# http-client generator’s “root output model” detection so streaming payload/event models are no longer treated as ClientResult-castable outputs, preventing generation of unusable public static explicit operator <Model>(ClientResult) API surface for SSE/JSON Lines streaming scenarios (fix for #11856).

Changes:

• Stop considering InputStreamingType.ValueType as a root ClientResult output model (so streaming item/event models don’t get ClientResult conversion operators).
• Add regression coverage ensuring streaming responses do not produce ClientResult explicit operators.
• Regenerate Spector and local streaming fixtures to reflect the removed operator(s).

File summaries

File	Description
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/ScmTypeFactory.cs	Removes streaming-type recursion when collecting RootOutputModels, preventing generation of ClientResult conversion operators for streaming payload models.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/DynamicModelSerializationTests.cs	Adds a regression test for SSE/JSONL streaming responses to ensure no explicit ClientResult operator is generated.
packages/http-client-csharp/generator/TestProjects/Spector/http/streaming/sse/src/Generated/Models/Info.Serialization.cs	Regenerated fixture: removes ClientResult explicit operator for SSE streaming model.
packages/http-client-csharp/generator/TestProjects/Spector/http/streaming/jsonl/src/Generated/Models/Info.Serialization.cs	Regenerated fixture: removes ClientResult explicit operator for JSONL streaming model.
packages/http-client-csharp/generator/TestProjects/Local/Sample-TypeSpec/src/Generated/Models/StreamingItem.Serialization.cs	Regenerated fixture: removes ClientResult explicit operator for a streaming payload model.

Review details

• Files reviewed: 2/5 changed files
• Comments generated: 1
• Review effort level: Lite

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