fix: auto-create intermediate directories for nested database paths

The path traversal validation introduced in e58189d canonicalizes the
parent directory to verify containment, but this fails with an IO error
when intermediate directories in a relative path (e.g. "subdir/mydb.db")
do not yet exist. The previous implementation returned the joined path
without requiring the parent to exist.

Call create_dir_all on the parent before canonicalizing so that nested
relative paths work without the caller needing to pre-create
subdirectories.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: pmorris-dev

src/resolve.rs

@@ -83,12 +83,18 @@ fn validate_and_resolve(path: &str, base: &Path) -> Result<PathBuf, Error> {
    let canonical_resolved = if joined.exists() {
       joined.canonicalize()
    } else {
-      // Canonicalize the parent and re-append the file name
-      joined
+      // Ensure intermediate directories exist so that canonicalize can resolve the
+      // parent. This matches the caller's expectation that nested relative paths like
+      // "subdir/mydb.db" work without pre-creating "subdir/".
+      let parent = joined
          .parent()
-         .ok_or_else(|| Error::InvalidPath("path has no parent".to_string()))?
+         .ok_or_else(|| Error::InvalidPath("path has no parent".to_string()))?;
+
+      create_dir_all(parent)?;
+
+      parent
          .canonicalize()
-         .map(|parent| parent.join(joined.file_name().unwrap_or_default()))
+         .map(|p| p.join(joined.file_name().unwrap_or_default()))
    }
    .map_err(|e| Error::InvalidPath(format!("cannot canonicalize path: {e}")))?;
 
@@ -135,10 +141,18 @@ mod tests {
    #[test]
    fn test_subdirectory_path() {
       let base = make_temp_base();
-      // Create the subdirectory so canonicalize can resolve it
-      fs::create_dir_all(base.join("subdir")).unwrap();
+      // Intermediate directories are auto-created — no manual setup needed
       let result = validate_and_resolve("subdir/mydb.db", &base).unwrap();
       assert_eq!(result, base.join("subdir/mydb.db"));
+      assert!(base.join("subdir").is_dir());
+   }
+
+   #[test]
+   fn test_nested_subdirectory_path() {
+      let base = make_temp_base();
+      let result = validate_and_resolve("a/b/c/mydb.db", &base).unwrap();
+      assert_eq!(result, base.join("a/b/c/mydb.db"));
+      assert!(base.join("a/b/c").is_dir());
    }
 
    #[test]