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fix(frame): keep nine-patch padding within the image limits

The frame filter grows the canvas by the padding it reads from the nine-patch
image without looking at MAX_WIDTH, MAX_HEIGHT or MAX_PIXELS, so with
MAX_WIDTH=16 a 16 pixel wide source framed by a nine-patch with 5 pixels of
horizontal padding came back 21 pixels wide. handle_padding now checks each
side the padding grows against those limits before allocating the new canvas
and answers 400 with the reason "Frame padding exceeds configured image
limits", leaving the engine image untouched. 0 turns any of the three checks
off, and so does None for MAX_PIXELS, the same way PILEngine ignores a zero
MAX_PIXELS.

Thumbor clamps oversized sources and requested sizes to MAX_WIDTH and
MAX_HEIGHT, but padding cannot shrink without distorting the frame, so the
guard rejects instead: a request whose output already sits at a limit now
fails when its frame pads that side, where it used to return an image past
the limit. Outputs thumbor already lets past a limit, such as an unclamped
proportional side or an upscale beyond MAX_PIXELS, keep passing unless the
frame pads the side that is over, or pads at all for MAX_PIXELS. MAX_PIXELS is
documented for images read by thumbor, and this applies it to the padded
canvas too. The change used to live in #1735 and moved here because it does
not depend on the Rust rewrite.

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