Validate that the file content has type bytes (pantsbuild#20261)

Fixes pantsbuild#20024.

src/python/pants/engine/fs.py

@@ -53,6 +53,12 @@ class FileContent:
     content: bytes
     is_executable: bool = False
 
+    def __post_init__(self):
+        if not isinstance(self.content, bytes):
+            raise TypeError(
+                f"Expected 'content' to be bytes, but got {type(self.content).__name__}"
+            )
+
     def __repr__(self) -> str:
         return (
             f"FileContent(path={self.path}, content=(len:{len(self.content)}), "

src/python/pants/engine/fs_test.py

@@ -142,6 +142,18 @@ def try_with_backoff(assertion_fn: Callable[[], bool], count: int = 4) -> bool:
     return False
 
 
+# -----------------------------------------------------------------------------------------------
+# `FileContent`
+# -----------------------------------------------------------------------------------------------
+
+
+def test_file_content_non_bytes():
+    with pytest.raises(TypeError) as exc:
+        FileContent(path="4.txt", content="four")
+
+    assert str(exc.value) == "Expected 'content' to be bytes, but got str"
+
+
 # -----------------------------------------------------------------------------------------------
 # `PathGlobs`, including `GlobMatchErrorBehavior` and symlink handling
 # -----------------------------------------------------------------------------------------------