Update CreateMessage call to use new overload with Uri, method, and classifier

[Copilot]

Updates the TypeSpec C# HTTP client generator to use the new CreateMessage overload introduced in System.ClientModel 1.6.0 that accepts Uri, HTTP method, and response classifier parameters.

Changes Made

System.ClientModel Version Update:

• Updated dependency from 1.5.0 to 1.6.0 in NewProjectScaffolding.cs

Generator Code Updates:

• Updated ClientPipelineApi.CreateMessage abstract signature to accept ValueExpression uri, ValueExpression method, and ValueExpression responseClassifier parameters
• Modified ClientPipelineProvider.CreateMessage implementation to pass these parameters to the underlying ClientPipeline.CreateMessage call
• Restructured RestClientProvider to build the URI first, then call CreateMessage with the complete URI and HTTP method

Code Generation Pattern Change:
The generated client code pattern has been updated:

Before:

PipelineMessage message = Pipeline.CreateMessage();
message.ResponseClassifier = PipelineMessageClassifier200;
PipelineRequest request = message.Request;
request.Method = "GET";
ClientUriBuilder uri = new ClientUriBuilder();
uri.Reset(_endpoint);
request.Uri = uri.ToUri();

After:

ClientUriBuilder uri = new ClientUriBuilder();
uri.Reset(_endpoint);
PipelineMessage message = Pipeline.CreateMessage(uri.ToUri(), "GET", PipelineMessageClassifier200);
message.ResponseClassifier = PipelineMessageClassifier200;
PipelineRequest request = message.Request;

Test Updates:

• Updated all affected test expectation files to match the new generated code pattern
• Fixed test mock classes to match the updated abstract method signature

This change enables the generator to take advantage of the more efficient CreateMessage overload while maintaining full compatibility with existing functionality including complex scenarios like pagination and parameter handling.

Fixes #8168.

Warning

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