a2a/x402: add AP2 mandate foundation for agent payments

Goal Add an opt-in AP2 mandate foundation so agents can carry auditable payment authorization over A2A and settle over x402 without changing defaults.

Scope

• Define minimal Checkout Mandate and Payment Mandate data structures with signing/verification helpers.
• Attach verified mandate metadata to A2A/x402 payment flows behind explicit options.
• Preserve current public defaults and avoid broad API breakage.
• Include docs/example notes only where needed to make the flow discoverable.

Acceptance criteria

• Unit tests cover mandate creation, signing, verification failure, and x402 rail metadata.
• A focused integration or gateway test shows an A2A request carrying mandate metadata into a paid-tool path.
• go test ./... passes.