fix: an empty proxy means a direct connection on all platforms

[orenjp]

sentry_options_set_proxy(options, "") is meant to turn the proxy off. Today it does so only where libcurl is used. Everywhere else, an empty proxy falls back to proxy discovery:

Transport	Proxy "" before	Proxy "" after
WinHTTP transport (Windows)	system proxy (WINHTTP_ACCESS_TYPE_AUTOMATIC_PROXY)	direct
curl transport (Linux, macOS)	direct	direct
crashpad handler, Linux	direct (<empty>)	direct
crashpad handler, Windows	system proxy	direct
crashpad handler, macOS	system proxy	direct
native backend daemon	http_proxy/https_proxy, then the system proxy on Windows	direct

The default (no sentry_options_set_proxy call) does not change.

This PR makes the following changes:

• WinHTTP transport: opens the session with WINHTTP_ACCESS_TYPE_NO_PROXY for an empty proxy.
• crashpad backend: passes <empty> to the handler on every platform, not only on Linux.
• native backend: the daemon applied the proxy only when it was not empty, so it could not tell "" from no proxy. The shared context now carries has_proxy.
• external/crashpad: points at fix: treat the <empty> proxy as a direct connection on Windows and macOS crashpad#175, which makes the Windows and macOS handlers treat <empty> as a direct connection.
• sentry.h: the sentry_options_set_proxy doc says what an empty string does.

Tests

The existing test_proxy_set_empty and test_crashpad_proxy_set_empty set only the proxy environment variables. The WinHTTP transport and the Windows and macOS handlers never read those, so both tests pass with or without this change.

The new system_proxy helper in tests/proxy.py sets the current user's system proxy to the test proxy and restores it afterwards: WinINet on Windows, networksetup (with sudo -n) on macOS. It skips on other platforms. These tests use it:

• test_proxy_system, for the WinHTTP transport on Windows.
• test_crashpad_proxy_system, on Windows and macOS.
• test_native_proxy_system, on Windows.

Each runs twice. With proxy_empty=False it checks that the request goes through the system proxy, which proves the setup works. With proxy_empty=True it checks that the proxy sees no request. test_native_proxy_set_empty is the environment-variable test the other backends already have.

The first commit adds only the tests, so CI shows them failing without the fix:

• Tests only: https://github.com/orenjp/sentry-native/actions/runs/39159242842
• Tests and fix: https://github.com/orenjp/sentry-native/actions/runs/39159245018

Related: getsentry/crashpad#176, a separate fix for the macOS proxy upload, which stops waiting after 10 seconds.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.69%. Comparing base (bbbd78c) to head (79a2a94).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2218      +/-   ##
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+ Coverage   75.61%   75.69%   +0.08%     
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  Files         103      103              
  Lines       28261    28263       +2     
  Branches     5179     5180       +1     
==========================================
+ Hits        21369    21394      +25     
+ Misses       5536     5524      -12     
+ Partials     1356     1345      -11     

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