Ensure consistent indentation when adding dependencies (#14991)

## Summary

The basic problem here is that when we had multiple items in an inline
array, and that array expanded to multiple lines, we accidentally
changed the indentation part-way through due to how prefixes work in the
TOML.

Here's Claude's explanation of the root cause, which I find pretty
decent:

```
  Here's what happened step by step:

  1. First item ("iniconfig"): Has empty prefix "" → indentation_prefix stays None → uses default 4 spaces
  2. Second item ("ruff"): Has empty prefix "" → indentation_prefix stays None → uses default 4 spaces
  3. Third item ("typing-extensions"): Has prefix " " (single space from inline format) → indentation_prefix becomes
  Some(" ") → uses only 1 space!

  This produced:
  [dependency-groups]
  dev = [
      "iniconfig>=2.0.0",
      "ruff",
   "typing-extensions",  # ← Only 1 space instead of 4!
  ]

  Why the Third Item Had a Different Prefix

  In inline arrays like ["ruff", "typing-extensions"], the items are separated by commas and spaces. When parsed by
  the TOML library:
  - "ruff" has no prefix (it comes right after [)
  - "typing-extensions" has a single space prefix (the space after the comma)

  The Fix

  Moving the indentation calculation outside the loop ensures it's calculated only once:

  // Calculate indentation ONCE before the loop
  if let Some(first_item) = deps.iter().next() {
      let decor_prefix = /* get prefix from first item */
      indentation_prefix = (!decor_prefix.is_empty()).then_some(decor_prefix.to_string());
  }

  // Now use the same indentation for ALL items
  for item in deps.iter_mut() {
      // Apply consistent indentation to every item
  }

  This ensures all items get the same indentation (4 spaces by default when converting from inline arrays), producing
   the correct output:

  [dependency-groups]
  dev = [
      "iniconfig>=2.0.0",
      "ruff",
      "typing-extensions",  # ← Correct 4-space indentation
  ]

  The bug only affected arrays being converted from inline to multiline format, where different items might have
  different residual formatting from their inline representation.
```

Closes #14961.

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

crates/uv-workspace/src/pyproject_mut.rs

@@ -1562,29 +1562,29 @@ fn reformat_array_multiline(deps: &mut Array) {
 
     let mut indentation_prefix = None;
 
+    // Calculate the indentation prefix based on the indentation of the first dependency entry.
+    if let Some(first_item) = deps.iter().next() {
+        let decor_prefix = first_item
+            .decor()
+            .prefix()
+            .and_then(|s| s.as_str())
+            .and_then(|s| s.lines().last())
+            .unwrap_or_default();
+
+        let decor_prefix = decor_prefix
+            .split_once('#')
+            .map(|(s, _)| s)
+            .unwrap_or(decor_prefix);
+
+        indentation_prefix = (!decor_prefix.is_empty()).then_some(decor_prefix.to_string());
+    }
+
+    let indentation_prefix_str = format!("\n{}", indentation_prefix.as_deref().unwrap_or("    "));
+
     for item in deps.iter_mut() {
         let decor = item.decor_mut();
         let mut prefix = String::new();
 
-        // Calculate the indentation prefix based on the indentation of the first dependency entry.
-        if indentation_prefix.is_none() {
-            let decor_prefix = decor
-                .prefix()
-                .and_then(|s| s.as_str())
-                .and_then(|s| s.lines().last())
-                .unwrap_or_default();
-
-            let decor_prefix = decor_prefix
-                .split_once('#')
-                .map(|(s, _)| s)
-                .unwrap_or(decor_prefix);
-
-            indentation_prefix = (!decor_prefix.is_empty()).then_some(decor_prefix.to_string());
-        }
-
-        let indentation_prefix_str =
-            format!("\n{}", indentation_prefix.as_deref().unwrap_or("    "));
-
         for comment in find_comments(decor.prefix()).chain(find_comments(decor.suffix())) {
             match comment.comment_type {
                 CommentType::OwnLine => {

crates/uv/tests/it/edit.rs

@@ -13595,3 +13595,58 @@ fn add_path_outside_workspace_no_default() -> Result<()> {
 
     Ok(())
 }
+
+/// See: <https://github.com/astral-sh/uv/issues/14961>
+#[test]
+fn add_multiline_indentation() -> 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"
+
+        [dependency-groups]
+        dev = ["ruff", "typing-extensions"]
+    "#})?;
+
+    uv_snapshot!(context.filters(), context.add().arg("iniconfig").arg("--dev"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 4 packages in [TIME]
+    Prepared 3 packages in [TIME]
+    Installed 3 packages in [TIME]
+     + iniconfig==2.0.0
+     + ruff==0.3.4
+     + typing-extensions==4.10.0
+    ");
+
+    let pyproject_toml = context.read("pyproject.toml");
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            pyproject_toml, @r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+
+        [dependency-groups]
+        dev = [
+            "iniconfig>=2.0.0",
+            "ruff",
+            "typing-extensions",
+        ]
+        "#
+        );
+    });
+
+    Ok(())
+}