[http-client-csharp] Fix explode object query parameter serialization

[JoshLove-msft]

Fixes #11123

Problem

A model-typed query parameter marked with @query(#{ explode: true }) was serialized via the object's ToString, producing the type name in the URL:

uri.AppendQuery("filter", TypeFormatters.ConvertToString(filter), true);
// => GET /api/v1/items?filter=Example.FilterOptions

Fix

RestClientProvider.BuildAppendQueryStatement now expands a model-typed explode query parameter into one query entry per property, using each property's wire name (RFC 6570 form explode):

uri.AppendQuery("field", filter.Field, true);
uri.AppendQuery("value", filter.Value, true);
// => GET /parameters/query/explode/object?field=status&value=active

Models whose properties are all simple scalars/enums are expanded. Nested objects and collections fall back to the previous behavior pending a separate design decision (they are not defined by RFC 6570 form explode).

Tests

• Added TestBuildCreateRequestMethodWithExplodedModelQueryParameter unit test.
• All 70 RestClientProviderTests pass (no regressions).

The end-to-end Spector scenario and C# Spector test that exercise this against a live mock server are in a follow-up PR (the shared @typespec/http-specs scenario + generated client).

[pkg-pr-new]

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npm i https://pkg.pr.new/@typespec/http-client-csharp@11124

commit: b283582

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