#25548 fix(csharp): preserve operation server paths

Description

Fixes #25546

For C# generichost operations with their own server, append the operation path to the server base path and preserve an explicit port. Path parameter substitution remains in place.

Tests

• Added a focused generator regression test with a server base path, port, and path parameter.
• Verified a generated client sends https://example.com:8443/base/files/hello%20world through a local handler; before the fix it threw UriFormatException.
• Regenerated the C# generichost samples and built the .NET 10 Petstore sample.
• Verified the full public Dropbox C# client builds and uses the expected upload/download paths with this change and #25547's fix applied locally.

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Summary by cubic

Fixes C# generichost operations with per-operation servers so the operation path is appended to the server base path and explicit ports are preserved, which previously caused UriFormatException at runtime.

• Uses Host and Port separately instead of Authority, which lost explicit ports and produced invalid URIs.
• Appends the operation path to the server base path, keeping the server's base path intact.
• Adds a regression test covering a server with a base path, port, and path parameter, and regenerates all C# generichost samples.