Clarify system Python discovery logging order (#16463)

Resolves https://github.com/astral-sh/uv/issues/16453

When `--system` is used, the debug log now reports interpreter discovery
sources in the same order they are probed, prioritising the PATH ahead
of managed installs on every platform. I also added a few unit tests
that use `DiscoveryPreferences::sources`, ensuring the log strings stay
aligned with the actual discovery sequence for both System and
OnlySystem preferences.

crates/uv-python/src/discovery.rs

@@ -3167,7 +3167,7 @@ impl PythonPreference {
     fn sources(self) -> &'static [PythonSource] {
         match self {
             Self::OnlyManaged => &[PythonSource::Managed],
-            Self::Managed | Self::System => {
+            Self::Managed => {
                 if cfg!(windows) {
                     &[
                         PythonSource::Managed,
@@ -3178,9 +3178,20 @@ impl PythonPreference {
                     &[PythonSource::Managed, PythonSource::SearchPath]
                 }
             }
+            Self::System => {
+                if cfg!(windows) {
+                    &[
+                        PythonSource::SearchPath,
+                        PythonSource::Registry,
+                        PythonSource::Managed,
+                    ]
+                } else {
+                    &[PythonSource::SearchPath, PythonSource::Managed]
+                }
+            }
             Self::OnlySystem => {
                 if cfg!(windows) {
-                    &[PythonSource::Registry, PythonSource::SearchPath]
+                    &[PythonSource::SearchPath, PythonSource::Registry]
                 } else {
                     &[PythonSource::SearchPath]
                 }
@@ -3345,7 +3356,9 @@ mod tests {
     };
     use uv_platform::{Arch, Libc, Os};
 
-    use super::{Error, PythonVariant};
+    use super::{
+        DiscoveryPreferences, EnvironmentPreference, Error, PythonPreference, PythonVariant,
+    };
 
     #[test]
     fn interpreter_request_from_str() {
@@ -3591,6 +3604,36 @@ mod tests {
         );
     }
 
+    #[test]
+    fn discovery_sources_prefer_system_orders_search_path_first() {
+        let preferences = DiscoveryPreferences {
+            python_preference: PythonPreference::System,
+            environment_preference: EnvironmentPreference::OnlySystem,
+        };
+        let sources = preferences.sources(&PythonRequest::Default);
+
+        if cfg!(windows) {
+            assert_eq!(sources, "search path, registry, or managed installations");
+        } else {
+            assert_eq!(sources, "search path or managed installations");
+        }
+    }
+
+    #[test]
+    fn discovery_sources_only_system_matches_platform_order() {
+        let preferences = DiscoveryPreferences {
+            python_preference: PythonPreference::OnlySystem,
+            environment_preference: EnvironmentPreference::OnlySystem,
+        };
+        let sources = preferences.sources(&PythonRequest::Default);
+
+        if cfg!(windows) {
+            assert_eq!(sources, "search path or registry");
+        } else {
+            assert_eq!(sources, "search path");
+        }
+    }
+
     #[test]
     fn interpreter_request_to_canonical_string() {
         assert_eq!(PythonRequest::Default.to_canonical_string(), "default");