THRIFT-6045: Add recursion-depth round-trip test for the Ruby library

[Jens-G]

THRIFT-6045: Add a recursion-depth round-trip test for the Ruby library

This is a test-only change — it adds the regression test without the implementation; the limit itself is left to a follow-up (see the discussion below).

spec/recursion_depth_spec.rb drives the generated read/write path (Thrift::Struct#read/#write, Thrift::Union#read/#write) over recursive RecTree (struct), RecUnion (union) and RecError (exception) types added to ThriftSpec.thrift, across BinaryProtocol and (when the native extension is loaded) BinaryProtocolAccelerated.

• Active (round-trip / within-limit): a chain at the limit round-trips, a wide shallow tree round-trips, a struct through a MultiplexedProtocol, and the exception round-trip.
• Pending (limit-enforcement, over-limit write/read for struct, union and exception): the Ruby library does not bound recursion depth yet, so these are expected to fail until the limit is implemented, at which point RSpec flags them to be enabled.

The over-limit read payloads are hand-serialized with the real recursive field (id 1, list<self>) so a future guarded reader recurses through the generated path rather than skip().

Validated locally (Ruby 3.0.2, pure-Ruby): 10 examples, 0 failures, 5 pending. Implementation note for the follow-up: the native thrift_native C extension rebinds Thrift::Struct#read/#write, so a complete fix must also guard rb_thrift_struct_write/read (and the union equivalents) in C, not only the pure-Ruby path.

🤖 Generated with Claude Code

[kpumuk]

This PR is insufficient:

• It only covers pure Ruby implementation, and not the native extension
• It does not cover Union
• There are no tests

I am currently working on https://issues.apache.org/jira/browse/THRIFT-5938, with recursion depth enforcement being a part of the problem. I can extract my change to merge it ahead of the configuration class

[Jens-G]

This PR is insufficient:

It is way more than that. That#s hy I made it a draft. Different story.

I can extract my change to merge it ahead of the configuration class

Ok I only push the test then and leave it up to you, including the ticket.