fix: MacProxy crash in CFNumberGetValue on iOS

[nwestfall]

Fixes #134617

On iOS with CoreCLR, MacProxy crashes with EXC_BAD_ACCESS in CFNumberGetValue when the system proxy configuration returns an HTTP/HTTPS proxy, whether from a PAC script or a manual setting. SocketsHttpHandler goes through this path whenever it consults HttpClient.DefaultProxy. This was discovered while moving from .NET 10 to .NET 11 and testing with Browserstack (see issue for details).

Cause

• Natively, CFNumberType is declared as CF_ENUM(CFIndex, CFNumberType), so it's 64-bit. The P/Invoke in Interop.CoreFoundation.CFNumber.cs passes it as an int-sized enum, which leaves the upper 32 bits of x1 unspecified.
• CFNumberGetValue uses the whole register as an index into its type table (ldrh w9, [x9, x20, lsl #1]), so it reads far out of bounds.
• JIT and AOT code happen to zero-extend 32-bit values, which kept this latent.
• The CoreCLR interpreter loads native-call arguments from 8-byte stack slots whose upper half can still hold stale data. On iOS that turns the bad declaration into a crash.

Changes

• Interop.CoreFoundation.CFNumber.cs: the P/Invoke now takes CFIndex. A wrapper keeps the typed enum for callers.
• MacProxy.cs: GetProxy(Uri) now calls a new internal GetProxy(Uri, SafeCFDictionaryHandle) overload that takes the proxy settings, so tests can supply their own. Behavior is unchanged.
• System.Net.Http.Unit.Tests: on Apple targets, the project now compiles the real MacProxy and its interop instead of the fake, and adds MacProxyTest.
  • The tests create proxy settings in-process, in the format CFNetworkCopySystemProxySettings returns, so they don't depend on the machine's proxy configuration.

Testing

Please note, this does removes the existing Fakes/MacProxy.cs file as we need to use the real one for testing. I'm unsure if this is the best approach, but it got me to first replicate the failure I was encountering on an actual device.

The new tests can only fail where this code runs on the interpreter. Results on the iOS simulator with the 11.0 RC1 CoreCLR runtime pack:

Test	Before	After
GetProxy_HttpProxy_ReturnsProxyUri	SIGSEGV	pass
GetProxy_ProxyAutoConfigurationScript_ReturnsProxyUri	SIGSEGV	pass
GetProxy_ProxyAutoConfigurationScriptReturnsDirect_ReturnsNull	pass	pass
GetProxy_NoProxy_ReturnsNull	pass	pass

GetProxy_ProxyAutoConfigurationScriptReturnsDirect_ReturnsNull goes through the same PAC callback without reading a port, which shows the callback isn't the problem.

Other runs, all against the 11.0 RC1 runtime using this branch's System.Net.Http and test builds:

• System.Net.Http.Unit.Tests, full suite: passes on macOS and on the iOS simulator (interpreter).
• System.Net.Http.Functional.Tests, full inner-loop suite: passes on macOS, with results identical to the unfixed build.
• The unit test project builds for all eight of its target frameworks with no warnings.

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What changed in this PR

Fixes the Apple CFNumberGetValue ABI mismatch that can crash CoreCLR’s interpreter when resolving proxies.

Changes:

• Passes CFNumberType as pointer-sized CFIndex.
• Adds injectable proxy settings for deterministic testing.
• Replaces the fake proxy with real Apple interop tests.

File	Description
src/​libraries/​System.Net.Http/​tests/​UnitTests/​System.Net.Http.Unit.Tests.csproj	Updated as part of this pull request.
src/​libraries/​System.Net.Http/​tests/​UnitTests/​MacProxyTest.cs	Updated as part of this pull request.
src/​libraries/​System.Net.Http/​tests/​UnitTests/​Fakes/​MacProxy.cs	Updated as part of this pull request.
src/​libraries/​System.Net.Http/​src/​System/​Net/​Http/​SocketsHttpHandler/​MacProxy.cs	Updated as part of this pull request.
src/​libraries/​Common/​src/​Interop/​OSX/​Interop.CoreFoundation.CFNumber.cs	Updated as part of this pull request.

[nwestfall]

Closing in favor of #135089