fix: accept bounded large egress grant headers

[jacksonriding]

Signed file grants can exceed Bun's default 16 KiB request-header limit when an execution has many input files. Those requests receive HTTP 431 before the gateway can validate the grant.

Set a finite 256 KiB header limit in the bundled gateway image, dedicated production/development images, and package startup script. The regression exercises the deployed command: a 32 KiB grant is accepted while a request above 256 KiB remains rejected. It also checks all four startup commands.

Validation on Bun 1.4.2:

• Header-limit regression passes; the default command rejects the same 32 KiB request.
• docker buildx build --check passes for both changed Dockerfiles.

The larger finite cap increases the maximum header bytes accepted per request.