Re-test Flow A (pre-signed chunks) on the new staging

[crtahlin]

Part of #240. Depends on #SUB3 (gateway live on isolated testnet Bee).

Re-test Flow A — pre-signed chunk upload (#218) end-to-end on the new staging, now that staging has its own Bee (this finally removes the #236 ambiguity where a shared/under-connected node masked code behaviour).

Scope

• Using the existing tooling (tests/tools/swarm_stamp.py Stamper, EIP-191 signing), sign a chunk against a testnet batch and POST /api/v1/chunks/ to staging.
• Confirm the chunk is accepted by the network and retrievable (a non-deferred upload must not 201 unless the network accepted it) — verify via download, not just status code.
• Exercise the prepaid bandwidth credit path (/chunks/credit) if x402 is enabled on staging.

Acceptance criteria

• Signed chunk uploaded via /chunks is retrievable from the testnet network.
• Behaviour matches the safeguards (no false success on a chunk the network didn't accept).
• Any gap found is filed as a follow-up.