Add test for extra-in-constraint (#3231)

## Summary

We have the wrong behavior here, so starting by adding a test for it.

crates/uv/tests/pip_compile.rs

@@ -288,6 +288,54 @@ fn compile_constraints_markers() -> Result<()> {
     Ok(())
 }
 
+/// Resolve a package from a `requirements.in` file, with a `constraints.txt` file that uses an
+/// extra. The constraint should be enforced, but the extra should _not_ be included in the output
+/// (though it currently _is_ included).
+#[test]
+fn compile_constraint_extra() -> Result<()> {
+    let context = TestContext::new("3.12");
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str("flask")?;
+
+    // Constrain a transitive dependency based on the Python version
+    let constraints_txt = context.temp_dir.child("constraints.txt");
+    constraints_txt.write_str("flask[dotenv]<24.3.0")?;
+
+    uv_snapshot!(context.compile()
+            .arg("requirements.in")
+            .arg("--constraint")
+            .arg("constraints.txt"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] --exclude-newer 2024-03-25T00:00:00Z requirements.in --constraint constraints.txt
+    blinker==1.7.0
+        # via flask
+    click==8.1.7
+        # via flask
+    flask==3.0.2
+    itsdangerous==2.1.2
+        # via flask
+    jinja2==3.1.3
+        # via flask
+    markupsafe==2.1.5
+        # via
+        #   jinja2
+        #   werkzeug
+    python-dotenv==1.0.1
+        # via flask
+    werkzeug==3.0.1
+        # via flask
+
+    ----- stderr -----
+    Resolved 8 packages in [TIME]
+    "###
+    );
+
+    Ok(())
+}
+
 /// Resolve a package from an optional dependency group in a `pyproject.toml` file.
 #[test]
 fn compile_pyproject_toml_extra() -> Result<()> {