refactor(api): redundant ml_mfp check #26059

buf_ensure_loaded already checks `(buf->b_ml.ml_mfp != NULL)`. #25823

TODO:
- #10070 #13201 All buffer-related API functions except
  `nvim_buf_is_loaded` (and `nvim_buf_is_valid`?) should always call
  `buf_ensure_loaded`. Because the _common case_ is that plugins expect
  the buffer to "just work"—and for the uncomon, performance-sensitive
  case, the script can check `nvim_buf_is_loaded` to avoid implicitly
  loading a buffer.
- Update documentation to clarify the above semantics.

src/nvim/api/buffer.c

@@ -335,13 +335,11 @@ void nvim_buf_set_lines(uint64_t channel_id, Buffer buffer, Integer start, Integ
     return;
   }
 
-  // load buffer first if it's not loaded
+  // Load buffer if necessary. #22670
-  if (buf->b_ml.ml_mfp == NULL) {
   if (!buf_ensure_loaded(buf)) {
     api_set_error(err, kErrorTypeException, "Failed to load buffer");
     return;
   }
-  }
 
   bool oob = false;
   start = normalize_index(buf, start, true, &oob);
@@ -517,13 +515,11 @@ void nvim_buf_set_text(uint64_t channel_id, Buffer buffer, Integer start_row, In
     return;
   }
 
-  // load buffer first if it's not loaded
+  // Load buffer if necessary. #22670
-  if (buf->b_ml.ml_mfp == NULL) {
   if (!buf_ensure_loaded(buf)) {
     api_set_error(err, kErrorTypeException, "Failed to load buffer");
     return;
   }
-  }
 
   bool oob = false;