* fix(config): honor $VAR substitution for request_admission integer fields
AppConfig.resolve_env_variables replaces every `$VAR` reference with the raw
environment string. RequestAdmissionConfig.requests_per_minute and
max_queue_size are declared `strict=True` so a bool or float cannot slip in,
but strict mode also refuses the string "60", so `requests_per_minute: $RPM`
failed the whole config load with "Input should be a valid integer" even when
RPM=60 — contradicting config.example.yaml's promise that environment
variables work for every field. They were the only strict fields in the
config schema; every other integer coerces in lax mode.
Wrap both fields in a BeforeValidator that converts a decimal integer literal
arriving as a string and leaves everything else untouched, so the strict check
behind it still rejects bools, floats, "60.0" and non-numeric strings.
Tests pin the accepted literals, the rejections that must survive, and the
end-to-end path from a config.yaml `$VAR` reference through AppConfig.from_file.
* docs(changelog): reference #5838 in the request_admission entries
