feat: implement full fee payer support

[brendanjryan]

Port of tempoxyz/mpp-rs#71 to pympp.

Changes

Client side (client.py)

• When feePayer: true in challenge, use expiring nonces (nonce_key=U256::MAX, nonce=0) with valid_before=now+25s for replay protection
• Set fee_token=None and add placeholder fee_payer_signature (0x00)
• No on-chain nonce fetch needed in fee payer mode

Server side (intents.py)

• Add _cosign_as_fee_payer() method: decodes 0x76 tx, recovers sender via ecrecover, sets fee_token, co-signs with domain 0x78
• _verify_transaction() co-signs locally when fee_payer account is configured on the method, falls back to external fee payer service otherwise

Factory (client.py tempo())

• Add fee_payer parameter to tempo() and TempoMethod
• Wire up _method backlink so ChargeIntent can access fee_payer account

Testing

• All 234 unit tests pass
• Tested live against mpp.sh/api/ping/paid ✅
• Tested pympp client → pympp example server (no fee payer) ✅
• Tested pympp client → pympp server with fee payer co-signing on testnet ✅

Usage

# Server with local fee payer co-signing
server = Mpp.create(
    method=tempo(
        chain_id=42431,
        fee_payer=TempoAccount.from_env("FEE_PAYER_KEY"),
        currency=PATH_USD,
        recipient="0x...",
        intents={"charge": ChargeIntent()},
    ),
)

# Client (no changes needed — auto-detects feePayer from challenge)
method = tempo(
    account=TempoAccount.from_key("0x..."),
    intents={"charge": ChargeIntent()},
)

[brendanjryan]

can I tighten this up at all?