[http-client-csharp] Fix partial method customization parameter rebinding

[ArcturusZhang]

Problem

C# partial method signature customization can generate invalid or uncompilable code in some paths.

Observed issues:

1. Method modifiers can be emitted in invalid order when the custom partial declaration includes virtual, e.g. public partial virtual ... instead of public virtual partial ....
2. The generic late customization path can update the generated method signature with custom parameter names, while the already-built method body still references the old generated parameter names.
3. Async partial implementations need to preserve generated async because custom partial declarations cannot include async without a body.

Expected behavior

When a generated method matches a custom partial method declaration:

• emitted modifiers should be valid C#;
• generated bodies should reference the final parameter names;
• async generated implementations should remain async;
• optional parameter metadata should continue to come from the custom partial declaration.

Notes

This is separate from the broader parameter ownership cleanup tracked by #11244.

[github-actions]

🎯 Issue Triage

Bug report for the C# client emitter: partial method customization can produce invalid or uncompilable C# code across three distinct paths (modifier ordering, parameter name rebinding, and async propagation).

Category: Bug
Area: emitter:client:csharp

⚠️ No reproduction provided. A minimal repro would help diagnose this issue.

🔍 Analysis

Three distinct failure modes are reported:

1. Modifier ordering: When a custom partial declaration includes virtual, generated modifiers are emitted as public partial virtual ... instead of the valid public virtual partial .... C# requires partial to appear after access/inheritance modifiers.
2. Parameter name rebinding: The generic late-customization path applies custom parameter names to the method signature but the already-generated method body still uses the original generated parameter names, causing compilation errors.
3. Async propagation: Custom partial declarations cannot include async without a body; the generator must preserve async on the generated implementation even when the custom partial declaration omits it.

This issue is noted as separate from the broader parameter ownership cleanup tracked by #11244.

Affected component: packages/http-client-csharp — partial method generation / customization logic.

📝 Suggested next steps

• Fix modifier ordering: ensure partial is placed after access and inheritance modifiers when merging custom declarations.
• Fix parameter rebinding: after applying custom parameter names to the signature, update all references within the already-built method body.
• Fix async propagation: retain async on generated implementations even when the custom partial declaration omits it.
• Add regression tests for each of the three scenarios.

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