Azure.Sdk.Tools.TestProxy_1.0.0-dev.20260128.1

Multipart Performance Updates (#13595)

* do a ton less parsing than we needed to before (application scope of sanitizers)
* precache multipart bodies into trees
* process multipart trees versus memory at rest, materialize after. we see 6x to 7x speedup in sanitization of multipart bodies with this optimization (50->60 seconds for a test run down to < 10)
* further memory optimizations