[fix][elixir] simplify connection module

[efcasado]

Description

This PR simplifies the generated Connection module according to the discussion here. The new approach is more idiomatic and improves the maintainability of the generator.

Additionally, it resolves the issue reported in #20562 by removing the problematic function specification from the generated code. The issue can be reproduced using Stripe's OpenAPI specification, available here.

== Compilation error in file lib/connection.ex ==
** (MismatchedDelimiterError) mismatched delimiter found on lib/connection.ex:92:45:
    error: unexpected token: )
    │
  4 │ defmodule Stripe.Connection do
    │                             └ unclosed delimiter
 ...
 92 │   @spec new(String.t(), String.t()), options) :: Tesla.Env.client()
    │                                             └ mismatched closing delimiter (expected "end")
    │
    └─ lib/connection.ex:92:45
    (elixir 1.18.3) lib/kernel/parallel_compiler.ex:428: anonymous fn/5 in Kernel.ParallelCompiler.spawn_workers/8

With these changes, #15075 becomes unnecessary.

I considered targeting the 8.0.x branch, but since the Elixir generator is still in alpha and this change also fixes a known issue, I believe it makes more sense to target master directly. What do you think?

As usual, curious to hear your thoughts @mrmstn and @wing328 😊

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[wing328]

thanks for the pr

we can consider this a bug fix to start with.

if later users need the old behaviour, we can try to come up with a way to fallback