fix(agent_registry): authenticate transaction registration/completion and forbid self-rating (#77)

[Vyacheslav-Tomashevskiy]

Issue

Fixes #77 — [Security] agent_registry register_transaction and complete_transaction are unauthenticated, letting reputation be fabricated.

Problem

In contracts/agent_registry/src/agent.rs:

• register_transaction never called require_auth on anyone, so any caller could create AgentTransaction records with an attacker-chosen agent and parties.
• complete_transaction only compared the supplied agent to stored state but never called agent.require_auth(), so anyone could complete on the agent's behalf and bump completed_agreements. It also re-incremented completed_agreements on every repeated call.
• rate_agent had no rater != agent guard.

Combined, one actor could manufacture transactions, complete them, and rate any verified agent at will.

Fix

• register_transaction now takes a caller: Address, calls caller.require_auth(), and requires the caller to be the agent or one of the listed parties. parties must be non-empty, all distinct, and must not include the agent — parties are the counterparties who may later rate, so excluding the agent forecloses self-rating at the source (InvalidParties).
• complete_transaction now calls agent.require_auth() and rejects double-completion with TransactionAlreadyCompleted, so completed_agreements can be bumped at most once per transaction and only by the agent.
• rate_agent rejects rater == agent with SelfRatingNotAllowed (defense-in-depth alongside the register-time exclusion). The existing party / completed / already-rated checks are kept.

New errors: SelfRatingNotAllowed = 14, InvalidParties = 15, TransactionAlreadyCompleted = 16.

Acceptance criteria

• register_transaction authenticates a legitimate party and validates parties are distinct/real (non-empty, no duplicates, not the agent)
• complete_transaction calls require_auth on the agent
• rate_agent rejects self-rating and only allows a real party of a completed transaction to rate, once
• Tests assert a non-party cannot register, complete, or rate, and an agent cannot rate itself

Tests

8 new adversarial tests (caller auth required for register/complete; non-participant caller rejected; agent-in-parties / duplicate / empty parties rejected; party caller allowed; double-completion rejected); existing transaction tests updated to the new register_transaction signature.

cargo test -p agent_registry → 31 passed; 0 failed
cargo fmt --check → clean
cargo clippy --tests → no warnings on changed files

Scope / breaking change

register_transaction gains a leading caller: Address parameter (deliberate — required to authenticate the registrant). Out of scope per the issue: weighting reputation by transaction value, cross-contract verification.

RTC payout wallet: RTCd1554f0f35576faf01d386a6be1c947f560dd0b7

[Vyacheslav-Tomashevskiy]

Rebased onto latest main and resolved the conflicts that appeared after the recent merges:

• errors.rs: renumbered the new variants to SelfRatingNotAllowed = 19, InvalidParties = 20, TransactionAlreadyCompleted = 21 so they no longer collide with the pause/admin variants (14-18) that landed on main.
• agent.rs: kept both check_paused(env)? and the new caller.require_auth() in register_transaction/complete_transaction.
• Updated the corresponding should_panic codes in the tests.

cargo test -p agent_registry is green (31 passed). PR is now mergeable: clean.

RTC wallet for payout: RTCd1554f0f35576faf01d386a6be1c947f560dd0b7