fix(api): auto-load buffers in nvim_buf_set_* operations (#35046)

fix(api): auto-load buffers in nvim_buf_set_* operations

Problem: Setting marks, lines, or text on unloaded buffers fails because
ml_line_count is not accurate until the buffer is loaded.

Solution: Add require_loaded_buffer() helper and use it in write operations
(nvim_buf_set_lines, nvim_buf_set_text, nvim_buf_set_mark). These now
auto-load buffers as needed.

src/nvim/api/buffer.c

@@ -288,6 +288,22 @@ ArrayOf(String) nvim_buf_get_lines(uint64_t channel_id,
   return rv;
 }
 
+static buf_T *require_loaded_buffer(Buffer buffer, Error *err)
+{
+  buf_T *buf = find_buffer_by_handle(buffer, err);
+  if (!buf) {
+    return NULL;
+  }
+
+  // Load buffer if necessary
+  if (buf->b_ml.ml_mfp == NULL && !buf_ensure_loaded(buf)) {
+    api_set_error(err, kErrorTypeException, "Failed to load buffer");
+    return NULL;
+  }
+
+  return buf;
+}
+
 /// Sets (replaces) a line-range in the buffer.
 ///
 /// Indexing is zero-based, end-exclusive. Negative indices are interpreted
@@ -315,18 +331,12 @@ void nvim_buf_set_lines(uint64_t channel_id, Buffer buffer, Integer start, Integ
   FUNC_API_SINCE(1)
   FUNC_API_TEXTLOCK_ALLOW_CMDWIN
 {
-  buf_T *buf = find_buffer_by_handle(buffer, err);
+  buf_T *buf = require_loaded_buffer(buffer, err);
 
   if (!buf) {
     return;
   }
 
-  // Load buffer if necessary. #22670
-  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);
   end = normalize_index(buf, end, true, &oob);
@@ -480,17 +490,11 @@ void nvim_buf_set_text(uint64_t channel_id, Buffer buffer, Integer start_row, In
     replacement = scratch;
   }
 
-  buf_T *buf = find_buffer_by_handle(buffer, err);
+  buf_T *buf = require_loaded_buffer(buffer, err);
   if (!buf) {
     return;
   }
 
-  // Load buffer if necessary. #22670
-  if (!buf_ensure_loaded(buf)) {
-    api_set_error(err, kErrorTypeException, "Failed to load buffer");
-    return;
-  }
-
   bool oob = false;
 
   // check range is ordered and everything!
@@ -1112,7 +1116,7 @@ Boolean nvim_buf_set_mark(Buffer buffer, String name, Integer line, Integer col,
   FUNC_API_SINCE(8)
 {
   bool res = false;
-  buf_T *buf = find_buffer_by_handle(buffer, err);
+  buf_T *buf = require_loaded_buffer(buffer, err);
 
   if (!buf) {
     return res;

test/functional/api/buffer_spec.lua

@@ -863,6 +863,7 @@ describe('api/buf', function()
       local new_bufnr = fn.bufnr('set_lines', true)
       lua_or_rpc.nvim_buf_set_lines(new_bufnr, 0, -1, false, {})
       eq({ '' }, lua_or_rpc.nvim_buf_get_lines(new_bufnr, 0, -1, false))
+      eq(true, api.nvim_buf_is_loaded(new_bufnr))
     end)
   end)
 
@@ -1809,6 +1810,14 @@ describe('api/buf', function()
       eq({ 'one', 'two' }, get_lines(0, 2, true))
     end)
 
+    it('auto-loads unloaded buffer', function()
+      local new_bufnr = fn.bufnr('set_text', true)
+      eq(false, api.nvim_buf_is_loaded(new_bufnr))
+      api.nvim_buf_set_text(new_bufnr, 0, 0, 0, -1, { 'foo' })
+      eq(true, api.nvim_buf_is_loaded(new_bufnr))
+      eq({ 'foo' }, api.nvim_buf_get_lines(new_bufnr, 0, -1, false))
+    end)
+
     describe('handles topline', function()
       local screen
       before_each(function()
@@ -2375,6 +2384,13 @@ describe('api/buf', function()
     it('fails when invalid buffer number is used', function()
       eq(false, pcall(api.nvim_buf_set_mark, 99, 'a', 1, 1, {}))
     end)
+    it('auto-loads unloaded buffer', function()
+      local new_bufnr = fn.bufnr('set_mark', true)
+      eq(false, api.nvim_buf_is_loaded(new_bufnr))
+      eq(true, api.nvim_buf_set_mark(new_bufnr, 'A', 0, 0, {}))
+      eq(true, api.nvim_buf_is_loaded(new_bufnr))
+      eq({ 0, 0 }, api.nvim_buf_get_mark(new_bufnr, 'A'))
+    end)
   end)
 
   describe('nvim_buf_del_mark', function()