The batch UART read change in #13818 exposed a latent bug where the
UART parser could start accumulating bytes mid-frame after module
restart, causing an infinite cycle of "Max command length exceeded"
warnings that prevented initialization.
Add header validation for the first 4 bytes of each frame, ensuring
the parser only accumulates data starting from a valid DATA_FRAME_HEADER
(AA FF 03 00) or CMD_FRAME_HEADER (FD FC FB FA). Non-matching bytes are
discarded until a valid frame start is found.
Fixes#14131
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
