fix(mfe): properly handle versionless configuration (vercel#9945)

### Description

TSIA

We were requiring the version to exist in `VersionOnly` schema so we
could decide which config schema to use.

I'm assuming I broke this in
vercel#9883, but it's not immediately
obvious to me how.

### Testing Instructions

Added failing unit test for versionless `Config::load_from_dir` call

crates/turborepo-microfrontends/src/lib.rs

@@ -70,7 +70,7 @@ impl Config {
     pub fn from_str(input: &str, source: &str) -> Result<Self, Error> {
         #[derive(Deserializable, Default)]
         struct VersionOnly {
-            version: String,
+            version: Option<String>,
         }
         let (version_only, _errs) = biome_deserialize::json::deserialize_from_json_str(
             input,
@@ -80,9 +80,11 @@ impl Config {
         .consume();
 
         let version = match version_only {
-            Some(VersionOnly { version }) => version,
+            Some(VersionOnly {
+                version: Some(version),
+            }) => version,
             // Default to version 1 if no version found
-            None => "1".to_string(),
+            Some(VersionOnly { version: None }) | None => "1".to_string(),
         };
 
         let inner = match version.as_str() {
@@ -180,6 +182,14 @@ mod test {
         path.create_with_contents(r#"{"version": "1", "applications": {"web": {"development": {"task": "serve"}}, "docs": {}}}"#)
     }
 
+    fn add_no_version_config(dir: &AbsoluteSystemPath) -> Result<(), std::io::Error> {
+        let path = dir.join_component(DEFAULT_MICROFRONTENDS_CONFIG_V1);
+        path.ensure_dir()?;
+        path.create_with_contents(
+            r#"{"applications": {"web": {"development": {"task": "serve"}}, "docs": {}}}"#,
+        )
+    }
+
     fn add_v2_config(dir: &AbsoluteSystemPath) -> Result<(), std::io::Error> {
         let path = dir.join_component(DEFAULT_MICROFRONTENDS_CONFIG_V1);
         path.ensure_dir()?;
@@ -195,6 +205,7 @@ mod test {
     struct LoadDirTest {
         has_v1: bool,
         has_alt_v1: bool,
+        has_versionless: bool,
         pkg_dir: &'static str,
         expected_version: Option<FoundConfig>,
         expected_filename: Option<&'static str>,
@@ -211,6 +222,7 @@ mod test {
                 pkg_dir,
                 has_v1: false,
                 has_alt_v1: false,
+                has_versionless: false,
                 expected_version: None,
                 expected_filename: None,
             }
@@ -226,6 +238,11 @@ mod test {
             self
         }
 
+        pub const fn has_versionless(mut self) -> Self {
+            self.has_versionless = true;
+            self
+        }
+
         pub const fn expects_v1(mut self) -> Self {
             self.expected_version = Some(FoundConfig::V1);
             self
@@ -257,9 +274,16 @@ mod test {
         .with_filename(DEFAULT_MICROFRONTENDS_CONFIG_V1_ALT);
 
     const LOAD_NONE: LoadDirTest = LoadDirTest::new("web");
+
+    const LOAD_VERSIONLESS: LoadDirTest = LoadDirTest::new("web")
+        .has_versionless()
+        .expects_v1()
+        .with_filename(DEFAULT_MICROFRONTENDS_CONFIG_V1);
+
     #[test_case(LOAD_V1)]
     #[test_case(LOAD_V1_ALT)]
     #[test_case(LOAD_NONE)]
+    #[test_case(LOAD_VERSIONLESS)]
     fn test_load_dir(case: LoadDirTest) {
         let dir = TempDir::new().unwrap();
         let repo_root = AbsoluteSystemPath::new(dir.path().to_str().unwrap()).unwrap();
@@ -271,6 +295,9 @@ mod test {
         if case.has_alt_v1 {
             add_v1_alt_config(&pkg_path).unwrap();
         }
+        if case.has_versionless {
+            add_no_version_config(&pkg_path).unwrap();
+        }
 
         let config = Config::load_from_dir(repo_root, pkg_dir).unwrap();
         let actual_version = config.as_ref().map(|config| match &config.inner {