Fix: Respect user-defined proxy settings during redirects

[tboy1337]

Summary

This PR potentially fixes issue #6981 where it was reported that explicitly set proxy configurations were being overridden by environment proxy settings during HTTP redirects. When users intentionally set proxies={'http': None} to bypass environment-configured proxies, the redirect logic would ignore this and reapply environment proxy settings, causing unexpected behavior.

Problem

Previously, when following redirects, the rebuild_proxies() method would always check environment variables (HTTP_PROXY, HTTPS_PROXY, etc.) via resolve_proxies() when trust_env=True, even when the user had explicitly passed proxy settings (including None to bypass proxies). This caused the following issues:

1. Explicit proxy bypass ignored: Setting proxies={'http': None} to bypass environment proxies would work for the initial request but fail on redirects
2. User intent overridden: Environment variables would take precedence over explicit user configuration during redirect resolution
3. Unexpected failures: Requests would fail with proxy connection errors on redirects even when the user explicitly disabled proxy usage

Solution

This PR introduces a _user_explicitly_set_proxies flag that tracks whether the user has explicitly provided proxy configuration in the request call. The changes include:

1. Flag propagation: The Session.send() method detects when proxies are explicitly set (including when set to None) and passes the flag through the redirect resolution chain
2. Conditional trust_env: The rebuild_proxies() method now respects this flag and disables environment variable checking when the user has explicitly configured proxies
3. Preserved user intent: User-defined proxy settings now take precedence over environment variables throughout the entire redirect chain

Changes

src/requests/sessions.py

• Added _user_explicitly_set_proxies parameter to resolve_redirects() and rebuild_proxies() methods
• Modified Session.send() to detect explicit proxy configuration and pass the flag
• Updated rebuild_proxies() to conditionally disable trust_env when user explicitly set proxies

tests/test_requests.py

• Added test_proxy_bypass_on_redirect_with_explicit_none() to verify that explicit proxies={'http': None} bypasses environment proxy settings even during redirects

Testing

The new test case verifies the fix by:

1. Setting an invalid proxy in the environment (http_proxy=INVALID_PROXY)
2. Making a request with explicit proxies={'http': None, 'https': None}
3. Following a redirect and confirming the request succeeds (would fail if environment proxy was used)

Backward Compatibility

This change is fully backward compatible:

• Existing behavior is preserved when proxies are not explicitly set
• The trust_env flag continues to work as expected for environment-based proxy configuration
• Only affects cases where users explicitly provide proxy configuration, ensuring their intent is respected