feat(a2a): formalize agent-to-agent payment protocol spec + fix tests

[kite-builds]

Summary

Closes #2.

Three concrete things, all grounded in the actual code:

1. New RFC-style spec at docs/agent-payment-protocol.md

Formalizes what was implicit in src/payment_protocol.py:

• Every PaymentRequest field with type, required-ness, and meaning
• Canonical wire encoding (sort_keys=True + tight separators) so the JSON is byte-deterministic
• content_hash semantics — excludes created_at and status so two requests for the same payment intent hash identically
• State-machine + lifecycle diagrams (happy path + refund path)
• Multi-chain section (current: EIP-155 chain_id; future: CAIP-2 for non-EVM)
• Backwards-compat notes for the two behavior changes below

2. Fix content_hash determinism in src/payment_protocol.py

Previously the hash included created_at (Unix epoch from time.time()), so two PaymentRequest objects with identical payment data but different instantiation times produced different hashes. That contradicted the docstring's "content-based hash for integrity check" promise.

New behavior: content_hash excludes created_at and status. For replay protection, agents should use request_id (UUID) — that's what it's there for.

3. Fix parse_wei("N wei") in src/payment_protocol.py

multiplier = {
    "ETH": 10**18,
    "wei": 1,        # ← lowercase, .upper() returns "WEI", falls through to default
    "KETH": 10**21,
}.get(currency.upper(), 10**18)

parse_wei("1000000000000000000 wei") was returning 10^36, not 10^18. Fix: uppercase the dictionary keys.

4. Make tests/test_payment_protocol.py collectible

The file had def/events(self): (literal / in a method name) on line 56, which made the whole file a SyntaxError and prevented pytest from collecting any of the 10 tests in it. Fixed and added the missing MockEvents stub the file referenced.

Test results

$ python -m pytest tests/ --ignore=tests/test_nonce_manager.py
50 passed in 0.25s

test_nonce_manager.py has a pre-existing missing-dep issue (sortedcontainers not in any requirements file) that's outside this PR's scope.

What this PR does NOT cover

Issue #2 mentions "multi-chain support" as a goal. v1.0 of the spec explicitly captures EIP-155 chain_id and flags CAIP-2 chain identifiers as future work in §7 / §9. Implementing actual non-EVM settlement (Solana, Stellar) is a separate larger PR — happy to follow up if you want a v1.1 design proposal.