Fix a RecursionError that occurs with deeply-nested request bodies

[kurtmckee]

Note

This issue was discovered while testing a service release.

The first commit, which introduces a test that demonstrates the problem, takes over 7 seconds locally to eventually throw RecursionError.

The second commit, which eliminates recursion entirely, makes the test succeed in 0.04 seconds.

Fixed

• Fix a RecursionError that occurs with deeply-nested request bodies.

[sirosen]

Because this is exchanging a very small and simple bit of recursive code for a much larger body of work to "unroll the loop", I think it's appropriate that we make sure we're reshaping this into something which is

1. Maximally readable
2. Maximally performant

We're reducing readability (net-net), so we should put in the effort to get as much benefit from that trade as possible.

I opened #1424 with a benchmark that I used to validate my suggestions. I encourage its use here -- also feel free to merge or cherry-pick it into here and make further changes! -- to verify these changes:

• use a deque
• use comprehensions rather than iteratively building dicts and lists (even though this requires the addition of a new helper function)

When I measured on Python 3.12 and 3.14, I found rather dramatic improvements, especially in the worst-case type of testing.
I only explore a very small fraction of the possible space with the proposed microbenchmark, so it may be worth our time to enhance it further?