Create missing dir for `uv export`/ `uv pip compile` (#9648)

## Summary

Closes #9643.

I modified the `commit` fn so this applies to `uv compile --output-file`
too. But I can move it to the export module if we want to restrict this
to `uv export` only.

## Test Plan

`cargo test`

crates/uv/src/commands/mod.rs

@@ -231,6 +231,10 @@ impl<'a> OutputWriter<'a> {
     /// Commit the buffer to the output file.
     async fn commit(self) -> std::io::Result<()> {
         if let Some(output_file) = self.output_file {
+            if let Some(parent_dir) = output_file.parent() {
+                fs_err::create_dir_all(parent_dir)?;
+            }
+
             // If the output file is an existing symlink, write to the destination instead.
             let output_file = fs_err::read_link(output_file)
                 .map(Cow::Owned)

crates/uv/tests/it/export.rs

@@ -761,6 +761,78 @@ fn frozen() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn create_missing_dir() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = ["anyio==3.7.0"]
+
+        [build-system]
+        requires = ["setuptools>=42"]
+        build-backend = "setuptools.build_meta"
+        "#,
+    )?;
+
+    context.lock().assert().success();
+
+    uv_snapshot!(context.filters(), context.export()
+        .arg("--output-file")
+        .arg("requirements/requirements.txt"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv export --cache-dir [CACHE_DIR] --output-file requirements/requirements.txt
+    -e .
+    anyio==3.7.0 \
+        --hash=sha256:275d9973793619a5374e1c89a4f4ad3f4b0a5510a2b5b939444bee8f4c4d37ce \
+        --hash=sha256:eddca883c4175f14df8aedce21054bfca3adb70ffe76a9f607aef9d7fa2ea7f0
+    idna==3.6 \
+        --hash=sha256:9ecdbbd083b06798ae1e86adcbfe8ab1479cf864e4ee30fe4e46a003d12491ca \
+        --hash=sha256:c05567e9c24a6b9faaa835c4821bad0590fbb9d5779e7caa6e1cc4978e7eb24f
+    sniffio==1.3.1 \
+        --hash=sha256:2f6da418d1f1e0fddd844478f41680e794e6051915791a034ff65e5f100525a2 \
+        --hash=sha256:f4324edc670a0f49750a81b895f35c3adb843cca46f0530f79fc1babb23789dc
+
+    ----- stderr -----
+    Resolved 4 packages in [TIME]
+    "###);
+    //
+    // Read the file contents.
+    let contents = apply_filters(
+        fs_err::read_to_string(
+            context
+                .temp_dir
+                .child("requirements")
+                .child("requirements.txt"),
+        )
+        .unwrap(),
+        context.filters(),
+    );
+    insta::assert_snapshot!(contents, @r###"
+    # This file was autogenerated by uv via the following command:
+    #    uv export --cache-dir [CACHE_DIR] --output-file requirements/requirements.txt
+    -e .
+    anyio==3.7.0 \
+        --hash=sha256:275d9973793619a5374e1c89a4f4ad3f4b0a5510a2b5b939444bee8f4c4d37ce \
+        --hash=sha256:eddca883c4175f14df8aedce21054bfca3adb70ffe76a9f607aef9d7fa2ea7f0
+    idna==3.6 \
+        --hash=sha256:9ecdbbd083b06798ae1e86adcbfe8ab1479cf864e4ee30fe4e46a003d12491ca \
+        --hash=sha256:c05567e9c24a6b9faaa835c4821bad0590fbb9d5779e7caa6e1cc4978e7eb24f
+    sniffio==1.3.1 \
+        --hash=sha256:2f6da418d1f1e0fddd844478f41680e794e6051915791a034ff65e5f100525a2 \
+        --hash=sha256:f4324edc670a0f49750a81b895f35c3adb843cca46f0530f79fc1babb23789dc
+    "###);
+    Ok(())
+}
+
 #[test]
 fn non_project() -> Result<()> {
     let context = TestContext::new("3.12");