Ignore pre-release versions in uv init --script (#15747)

## Summary

Ignore managed pre-release versions of Python for purposes of creating a
`requires-python` constraint when running `uv init --script`. This makes
the behavior consistent with `uv init` for normal projects.

## Test Plan

Added a regression test that makes sure that the constraint is
`requires-python = ">=3.13"`, even though a pre-release version of 3.14
is installed.

crates/uv-python/src/discovery.rs

@@ -726,7 +726,7 @@ fn python_interpreters<'a>(
     cache: &'a Cache,
     preview: Preview,
 ) -> impl Iterator<Item = Result<(PythonSource, Interpreter), Error>> + 'a {
-    python_interpreters_from_executables(
+    let interpreters = python_interpreters_from_executables(
         // Perform filtering on the discovered executables based on their source. This avoids
         // unnecessary interpreter queries, which are generally expensive. We'll filter again
         // with `interpreter_satisfies_environment_preference` after querying.
@@ -760,7 +760,21 @@ fn python_interpreters<'a>(
     })
     .filter_ok(move |(source, interpreter)| {
         satisfies_python_preference(*source, interpreter, preference)
-    })
+    });
+
+    if std::env::var(uv_static::EnvVars::UV_INTERNAL__TEST_PYTHON_MANAGED).is_ok() {
+        Either::Left(interpreters.map_ok(|(source, interpreter)| {
+            // In test mode, change the source to `Managed` if a version was marked as such via
+            // `TestContext::with_versions_as_managed`.
+            if interpreter.is_managed() {
+                (PythonSource::Managed, interpreter)
+            } else {
+                (source, interpreter)
+            }
+        }))
+    } else {
+        Either::Right(interpreters)
+    }
 }
 
 /// Lazily convert Python executables into interpreters.