[Ruby] Stabilize Kiota Ruby generator for production use (Open Payments SDK)

[mahmudsudo]

Is your feature request related to a problem? Please describe the problem.

Summary

Kiota's Ruby target (generation, abstractions: preview; JSON serialization,
auth: done; HTTP: preview) is close to usable but marked "not actively worked
on internally" per the Ruby tracking project. I'd like to take it from preview
to stable, motivated by generating a production Ruby SDK for the Open Payments
API (openpayments.dev, JSON only ,so FORM/MULTIPART/TEXT serialization gaps
#2077/#3032/#1049 aren't blockers for this use case).

This aligns with the Interledger Foundation's SDK Grant program, which lists
Ruby as a high-priority open track :
interledger/Grants wiki: SDK-grant-program

Client library/SDK language

Ruby

Describe the solution you'd like

Proposed approach

• Generate a Ruby client from the Open Payments OpenAPI document using the
  current preview generator; catalog every failure/bug encountered
• Fix generator bugs blocking correct compilation/runtime behavior
• Validate the generated client end-to-end against Open Payments' public
  test environment (auth.interledger-test.dev)
• Bring generation + abstractions + HTTP from preview to stable, matching
  the bar set by Go/Python/PHP

Questions for maintainers

1. Is there a known list of blocking issues for Ruby preview→stable, beyond
   the three serialization gaps already tracked?
2. Any existing branch or WIP I should be aware of before starting?
3. Open to reviewing a draft PR built against a working Ruby SDK for Open Payments?

Additional context

No response