fix(x402): treat expires_at == 0 as the "no expiry" sentinel

[kite-builds]

Problem

PaymentOffer.is_expired() only special-cased None:

def is_expired(self) -> bool:
    if self.expires_at is None:
        return False
    return time.time() > self.expires_at

So an offer with expires_at == 0 evaluates time.time() > 0, which is always True → the offer reports itself expired, and X402Middleware._validate_offer() rejects it with Payment offer expired at 0.

Why 0 should mean "no expiry"

0 is the project's own documented "no expiry" sentinel on the ZAP wire format. From switchboard/zap_transport.py:

• schema comment: .uint64("expires_at") # 0 sentinel = "no expiry"
• encode: ob.set_uint64(f["expires_at"], offer.expires_at or 0) (None → 0)
• decode: expires_at=int(expires) if expires else None (0 → None)

So a never-expiring offer that round-trips through the binary path (None → 0) — or any 402 server that sends {"expiresAt": 0} to mean "never expires" — is then wrongly treated as expired on the JSON/header path. The two transports disagree.

Reproduce

from switchboard.x402_middleware import PaymentOffer
import json
offer = PaymentOffer.from_header(json.dumps(
    {"amount": "1000", "recipient": "0x1", "chainId": 1, "expiresAt": 0}))
assert offer.is_expired() is False  # currently True

Fix

Treat a falsy expires_at (None or 0) as "never expires", aligning the JSON/header path with the ZAP binary path. Behaviour for None and for positive timestamps is unchanged.

Tests

Adds two regression tests (the unit case + the _validate_offer end-to-end case). Full suite: 173 passed, 56 skipped.

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