fix: user-configured proxies being overridden by http_proxy/https_proxy env vars

[cristianchiriac]

Fixes #2398.

Bug

JIRA(..., proxies={...}) sets self._session.proxies on the ResilientSession, but ResilientSession.request() never forwards a proxies kwarg to requests.Session.request() — it relies on self.proxies being picked up as a fallback. That fallback only works for keys that are still missing by the time requests does its final merge.

Tracing through requests.Session.request():

1. proxies = proxies or {} — since no proxies kwarg was passed, this starts as an empty dict.
2. merge_environment_settings(url, proxies, ...) fills that empty dict from the http_proxy/https_proxy environment variables via proxies.setdefault(k, v), then calls proxies = merge_setting(proxies, self.proxies).
3. merge_setting(request_setting, session_setting) builds the result by copying session_setting (self.proxies, the configured value) first, then calling .update(request_setting) — so the request-level dict (now populated from the environment) overwrites self.proxies for any matching key.

Net effect: whenever http_proxy/https_proxy are set in the environment, they silently win over an explicitly configured proxies dict, even though the explicit configuration should be authoritative.

I confirmed this against the actual requests merge functions:

>>> s.merge_environment_settings('http://example.com', {}, None, None, None)['proxies']
OrderedDict([('http', 'localhost:3128'), ('https', 'localhost:3128')])   # env wins, scheme stripped

>>> s.merge_environment_settings('http://example.com', dict(s.proxies), None, None, None)['proxies']
OrderedDict([('http', 'http://localhost:3128'), ('https', 'http://localhost:3128')])   # configured value kept

Fix

_jira_prepare() now passes self.proxies through explicitly as the proxies kwarg (when set and not already overridden per-call), so it's already populated before merge_environment_settings()'s environment fallback runs — setdefault() then only fills genuinely missing keys instead of a dict that later overwrites the configured values.

Test plan

• Added test_configured_proxies_are_forwarded, verifying a session-level proxies dict is passed through to requests.Session.request() unchanged.
• Added test_no_proxies_kwarg_when_unconfigured, verifying no proxies kwarg is added when none was configured (preserving prior behavior/defaults for the common case).
• Verified the new tests fail against the unmodified code and pass with the fix.
• pytest tests/test_resilientsession.py: 37 passed.
• ruff check and ruff format --check on the changed files: clean.

[github-actions]

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