fix: send explicit callback error and correct DPoP nonce comments (H6, CC2-CC4)
- H6: handle_deep_link now sends CallbackAbandoned error when code/state params are missing instead of silently dropping sender
- CC2: Correct RFC section references for nonce mechanism (§8 for DPoP proof, §11.1 for deduplication)
- CC3: Update comment to accurately describe nonce as bounding jti-dedup window, not full deduplication
- CC4: Clarify that nonce validation applies only at token endpoint, not resource endpoints