Fix paid /claim attestation binding contract

[azender1]

This PR fixes the older paid /claim attestation-binding bug separately from PR #41.

Problem:

• attestation_gate.gate() expects a binding preimage shaped as {amount_usd, charge_ref, nonce, subject_did}
• paid /claim was instead passing {agent_id, action_type, scope, timestamp:""}
• any real attestation carrying a binding digest could therefore never match that claim input

Fix:

• add optional attestation_binding to the paid /claim request schema
• pass those exact four binding fields to the attestation gate
• optional attestation mode: if binding material is absent, explicitly SKIP with claim_binding_missing
• required attestation mode: if binding material is absent, DENY before creating a claim
• do not derive these fields from the x402 access payment because that payment is SafeAgent's service fee, not the guarded real-world charge/action

Backwards compatibility:

• existing callers are unchanged when attestation checking is disabled
• attestation_binding is optional
• /claim/test is untouched

Tests cover optional missing binding, required missing binding, and exact binding propagation to the gate.