bug: provider discovery tests fail when system providers are installed

[marcusburghardt]

Description

DiscoverProviders() in pkg/provider/discovery.go:35-59 unconditionally scans the hard-coded system directory (/usr/libexec/complytime/providers, defined as SystemProviderDir in internal/complytime/consts.go:72) after scanning the user-provided directory. There is no mechanism to disable or override the system directory path.

Tests that assume an empty or controlled provider set fail on machines with system-installed providers (e.g., from RPM packages).

Affected Tests

4 tests in pkg/provider/:

Test	File:Line	Expected	Actual
TestDiscoverProviders_EmptyDir	discovery_test.go:23	empty	3 system providers
TestDiscoverProviders_NonPrefixedExecutables	discovery_test.go:33	empty	3 system providers
TestDiscoverProviders_ValidProvider	discovery_test.go:43	1 mock provider	1 mock + 3 system
TestManager_EmptyProviderDir	manager_test.go:75	empty	2 loaded providers

Reproduction

Install provider RPMs (or manually place executables at /usr/libexec/complytime/providers/), then run:

go test -race -v -run "TestDiscoverProviders_EmptyDir|TestDiscoverProviders_NonPrefixedExecutables|TestDiscoverProviders_ValidProvider|TestManager_EmptyProviderDir" ./pkg/provider/...

All four tests fail. CI does not reproduce this because CI runners have no system-installed providers.

Possible Fix

Add a mechanism to override or disable the system provider directory during tests, so discovery can be fully isolated from the host environment. The codebase already uses env var overrides for similar paths (XDG_CACHE_HOME, XDG_DATA_HOME, COMPLYTIME_WORKSPACE), which could serve as a pattern.