fix(buffer): enable history navigation when accept_handler keeps text (#1185)

[aoright]

Problem

Fixes #1185.
When accept_handler returns True (keep_text), the buffer was previously bypassed from calling self.reset(). While this succeeded in preserving the text in the buffer, it had severe unintended side effects:

1. _load_history_task was never reset to None, so newly appended history entries from self.append_to_history() were never loaded into self._working_lines.
2. As a result, subsequent calls to history_backward() / history_forward() (using the Up/Down arrow keys) failed to navigate any history entries.
3. Transient states such as validation errors and selection states were not cleared.

Solution

1. In validate_and_handle, when keep_text is True, call self.reset(document=Document(self.text, self.cursor_position)). This resets transient states and clears _load_history_task so history can reload on the next render.
2. In load_history, avoid duplicating the working line when it is already identical to the most recent history item.
3. Added an async unit test in tests/test_buffer.py to verify that history navigation works as expected across multiple accepts with keep_text=True.