feat(http-client-csharp): support partial method customization

[JoshLove-msft]

Adds support for customizing the signature of a generated client method (protocol or convenience) by declaring a partial method in custom code. The generator detects the partial declaration and emits a matching partial implementation, applying the user''s modifiers and parameter names while keeping the generated body.

Based on (and adapted from) #8627. Refs #10526.

Example

Custom code:

public partial class TestClient
{
    public partial ClientResult HelloAgain(BinaryContent content, RequestOptions options);
}

Generated code:

public partial class TestClient
{
    public partial ClientResult HelloAgain(BinaryContent content, RequestOptions options)
    {
        Argument.AssertNotNull(content, nameof(content));
        using PipelineMessage message = CreateRequest(content, options);
        return ClientResult.FromResponse(Pipeline.ProcessMessage(message, options));
    }
}

Scope

This feature is targeted at client methods (protocol and convenience methods on the generated *Client class). For other generated members (models, serialization, model factories), use the existing [CodeGenSuppress] + hand-written replacement pattern.

What is supported

• Changing modifiers (e.g., public → internal, adding virtual/override).
• Renaming parameters (e.g., p1 → content). The generator clones the parameter providers with the customer-chosen names while preserving all metadata (Location, WireInfo, SpreadSource, InputParameter, …) so the generated body keeps compiling.
• Both protocol and convenience method overloads (sync and async). Each overload is matched independently.

What is NOT supported

• Renaming the method itself. Matching is by (method name, ordered parameter type list). A renamed partial decl will not match any generated method. Use [CodeGenSuppress] + a hand-written method to rename, or follow up with an attribute-based opt-in (e.g., [CodeGenName("Get")]) in a future PR.
• Changing parameter types. Matching requires identical parameter type names; substituting a different type (even an implicitly convertible one) will simply fail to match and the partial decl will be ignored.
• Adding/removing/reordering parameters. The parameter type list must match the generated signature exactly.
• Default values on the partial implementation. C# requires partial method impls to have all parameters required; defaults are stripped on the impl (callers still see the defaults from the partial decl in custom code).

Changes

• MethodProvider.IsPartialMethod — tracks partial methods.
• NamedTypeSymbolProvider — detects partial method declarations (partial keyword + no body) via Roslyn syntax and sets the Partial modifier on the symbol-based MethodProvider.
• TypeProvider.FilterCustomizedMethods — when a generated method matches a custom partial declaration, emits a partial implementation using the custom signature + generated body, with all params forced to required. Skips its scan entirely when there are no custom methods (no extra allocations on the common path).
• TypeProvider.ShouldGenerate(MethodProvider) — partial declarations no longer suppress the generated method.
• ScmMethodProviderCollection.BuildProtocolMethod and BuildConvenienceMethod — perform the same matching before the body is built, so the body references the customer''s parameter names. Also fixes a gap in the original PR where the non-paging body still passed the generator''s parameters to CreateRequest(...).
• New PartialMethodCustomization static helper class (in Microsoft.TypeSpec.Generator) exposing TryFindCustomSignature, RenameAndCloneParameters, and BuildPartialSignature so downstream emitters (e.g. the management emitter, which wraps ClientProviders in its own public-surface providers) can reuse the same matching/cloning behavior in their own method builders. Throws on multiple partial decls matching the same generated method (defensive — C# itself disallows duplicate signatures).

Documentation

• Adds a "Customize a generated client method''s signature" section to the customization guide (packages/http-client-csharp/.tspd/docs/customization.md).

Tests

• CanCustomizeMethodSignature — protocol method parameter rename.
• CanCustomizeMethodModifierOnly — modifier-only customization with no parameter renames.
• CanCustomizeConvenienceMethodSignature — convenience method parameter rename (covers the BuildConvenienceMethod matching/cloning path).
• All existing generator (1434) and ClientModel (1316) tests continue to pass.

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