Redact new index credentials in `uv add` (#10329)

## Summary

Closes https://github.com/astral-sh/uv/issues/10328.

crates/uv-workspace/src/pyproject_mut.rs

@@ -303,7 +303,7 @@ impl PyProjectTomlMut {
             .and_then(|url| Url::parse(url).ok())
             .is_none_or(|url| CanonicalUrl::new(&url) != CanonicalUrl::new(index.url.url()))
         {
-            let mut formatted = Formatted::new(index.url.to_string());
+            let mut formatted = Formatted::new(index.url.redacted().to_string());
             if let Some(value) = table.get("url").and_then(Item::as_value) {
                 if let Some(prefix) = value.decor().prefix() {
                     formatted.decor_mut().set_prefix(prefix.clone());

crates/uv/tests/it/edit.rs

@@ -7039,6 +7039,93 @@ fn add_default_index_url() -> Result<()> {
 fn add_index_credentials() -> Result<()> {
     let context = TestContext::new("3.12");
 
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = []
+    "#})?;
+
+    // Provide credentials for the index via the environment variable.
+    uv_snapshot!(context.filters(), context.add().arg("iniconfig==2.0.0").env(EnvVars::UV_DEFAULT_INDEX, "https://public:heron@pypi-proxy.fly.dev/basic-auth/simple"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 2 packages in [TIME]
+    Prepared 1 package in [TIME]
+    Installed 1 package in [TIME]
+     + iniconfig==2.0.0
+    "###);
+
+    let pyproject_toml = fs_err::read_to_string(context.temp_dir.join("pyproject.toml"))?;
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            pyproject_toml, @r###"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = [
+            "iniconfig==2.0.0",
+        ]
+
+        [[tool.uv.index]]
+        url = "https://pypi-proxy.fly.dev/basic-auth/simple"
+        default = true
+        "###
+        );
+    });
+
+    let lock = fs_err::read_to_string(context.temp_dir.join("uv.lock"))?;
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            lock, @r###"
+        version = 1
+        requires-python = ">=3.12"
+
+        [options]
+        exclude-newer = "2024-03-25T00:00:00Z"
+
+        [[package]]
+        name = "iniconfig"
+        version = "2.0.0"
+        source = { registry = "https://pypi-proxy.fly.dev/basic-auth/simple" }
+        sdist = { url = "https://pypi-proxy.fly.dev/basic-auth/files/packages/d7/4b/cbd8e699e64a6f16ca3a8220661b5f83792b3017d0f79807cb8708d33913/iniconfig-2.0.0.tar.gz", hash = "sha256:2d91e135bf72d31a410b17c16da610a82cb55f6b0477d1a902134b24a455b8b3", size = 4646 }
+        wheels = [
+            { url = "https://pypi-proxy.fly.dev/basic-auth/files/packages/ef/a6/62565a6e1cf69e10f5727360368e451d4b7f58beeac6173dc9db836a5b46/iniconfig-2.0.0-py3-none-any.whl", hash = "sha256:b6a85871a79d2e3b22d2d1b94ac2824226a63c6b741c88f7ae975f18b6778374", size = 5892 },
+        ]
+
+        [[package]]
+        name = "project"
+        version = "0.1.0"
+        source = { virtual = "." }
+        dependencies = [
+            { name = "iniconfig" },
+        ]
+
+        [package.metadata]
+        requires-dist = [{ name = "iniconfig", specifier = "==2.0.0" }]
+        "###
+        );
+    });
+
+    Ok(())
+}
+
+#[test]
+fn existing_index_credentials() -> Result<()> {
+    let context = TestContext::new("3.12");
+
     let pyproject_toml = context.temp_dir.child("pyproject.toml");
     pyproject_toml.write_str(indoc! {r#"
         [project]