THRIFT-2462: Add netstd recursion-depth round-trip regression test

[Jens-G]

Summary

Validation of the recursion-depth handling for netstd showed that the limit is already enforced on the real (generated-code) read/write path: the netstd code generator emits IncrementRecursionDepth() / DecrementRecursionDepth() around the body of every generated WriteAsync / ReadAsync (see compiler/cpp/src/thrift/generate/t_netstd_generator.cc). Nested structs — both known fields (generated code) and unknown fields (TProtocolUtil.SkipAsync) — are therefore already bounded by TConfiguration.RecursionLimit (default 64).

An earlier revision of this PR additionally called IncrementRecursionDepth / DecrementRecursionDepth inside the protocols' WriteStructBegin/End and ReadStructBegin/End. Because the generated code already does this, that counted every struct twice, halving the effective limit and rejecting valid payloads at roughly limit/2. That change has been dropped — the three protocol files are now unchanged from master.

What remains is a round-trip regression test that exercises the limit through the generated WriteAsync / ReadAsync path (not the protocol *StructBegin/End methods in isolation), modeled on the Delphi test added in 1ffdcf2.

Changes

• lib/netstd/Tests/Thrift.Tests/Protocols/TProtocolRecursionDepthTests.cs — new round-trip test over the mutually recursive CoRec / CoRec2 chain and the wide RecTree from test/Recursive.thrift, for Binary / Compact / JSON:
  • below-limit and at-limit chains round-trip successfully (off-by-one guard),
  • over-limit chains are rejected on both write and read with TProtocolException(DEPTH_LIMIT),
  • a wide tree of siblings round-trips — guards correct DecrementRecursionDepth,
  • a cyclic object graph is rejected instead of recursing forever.
• Thrift.Compile.net10.csproj — generate test/Recursive.thrift so the recursive types are available to the test assembly.

No production code changes.

Test plan

• dotnet test (Thrift.Tests): 18 / 18 pass.
• Confirmed the test guards the real behavior: re-applying the earlier double-counting protocol change makes 9 / 18 fail (the below-/at-limit round-trips); with master's protocol code all 18 pass.

Co-authored with Claude Opus 4.8.