fix: changing block type over a multi-block selection destroys text

[matanrotman]

What's wrong

Selecting several blocks and toggling a heading — or switching them to Text — leaves every block holding the first block's text. The other blocks' content is overwritten and gone.

Cause

One line, in three files:

final delta = (node.delta ?? Delta()).toJson();   // read once: the first block
editorState.formatNode(selection, (node) => node.copyWith(
  attributes: {blockComponentDelta: delta},       // written into EVERY block
));

formatNode runs its callback for every node in the selection, so hoisting the delta out of the callback stamps one block's text over all the others. This PR reads the delta from the node actually being transformed.

Two details that make it easier to hit than it looks

• heading_toolbar_items.dart and paragraph_toolbar_item.dart also captured selection at widget build time, so a press could apply to a selection that no longer existed. Both now read editorState.selection when pressed. This is why onlyShowInSingleSelectionAndTextType doesn't prevent it — the guard is evaluated at build time, against the old selection.
• heading_command_shortcut.dart has no single-selection gate at all, so the heading keyboard shortcuts apply to a selection of any size.

Test

Added to test/new/block_component/heading_block_component/heading_command_shortcut_test.dart. On current main it reports:

Expected: ['first', 'second', 'third']
  Actual: ['first', 'first', 'first']

and passes with the fix.

Verification, honestly

The failing-then-passing run was done against a checkout whose heading_command_shortcut.dart is byte-identical to this branch's base, on Flutter 3.27.4. I could not compile this branch locally — appflowy_editor on main requires Flutter >=3.32.0 — so CI here is the first real compile. dart format is clean on all four files.

Context

Found after it destroyed a paragraph in a real document. The same mistake exists twice more in AppFlowy-IO/AppFlowy (text_heading_toolbar_item.dart, heading_toolbar_item.dart); a companion PR covers those.