THRIFT-6104: Accept Ruby Set subclasses in native struct writing

[kpumuk]

The native struct writer checked for the exact Set class when serializing Thrift set fields. A Set subclass was therefore routed to Hash handling and rejected, while the pure-Ruby writer accepted and serialized the same value.

The native writer now honors the public Set contract, allowing subclasses to serialize the same elements as an equivalent Set.

• Did you create an Apache Jira ticket? THRIFT-6104
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

[Copilot]

Pull request overview

This PR aligns the Ruby native struct writer’s handling of Thrift set fields with Ruby’s Set contract by accepting Set subclasses (previously rejected by a strict class check). This brings behavior in line with the pure-Ruby writer and avoids incorrectly routing Set subclasses through Hash handling.

Changes:

• Update native TTYPE_SET serialization to treat any Set subclass as a Set via rb_obj_is_kind_of.
• Add an RSpec example ensuring a Set subclass serializes identically to an equivalent Set.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/rb/spec/struct_spec.rb	Adds a regression spec asserting Set subclasses serialize like Set.
lib/rb/ext/struct.c	Changes native set-container detection from exact-class match to kind_of?(Set) semantics.

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