Idempotency conflict returns `FAILED_PRECONDITION` instead of `DUPLICATE_KEY` (§7.2, §12)

[nficano]

Category: spec-conformance Severity: major
Location: crates/arcp-runtime/src/runtime/server.rs:784-797; crates/arcp-core/src/error.rs:33-106
Spec: ARCP v1.1 §7.2 and §12

What

§7.2: "A reused key with conflicting parameters returns DUPLICATE_KEY." §12 lists DUPLICATE_KEY as a normative error code. The runtime instead emits job.failed with ErrorCode::FailedPrecondition, and the ErrorCode enum has no DuplicateKey variant at all. A conformant client matching on DUPLICATE_KEY will never see it. (This is distinct from issue #59, which concerns whether idempotency is enforced; enforcement now exists via idempotency_index, but the error code is wrong.)

Evidence

let mut err = Envelope::new(MessageType::JobFailed(JobFailedPayload {
    code: ErrorCode::FailedPrecondition,
    retryable: Some(false),
    message: format!(
        "idempotency key {} already bound to a different command intent",
        scope.idempotency_key
    ),
    details: None,
}));

The ErrorCode enum (error.rs) contains no DUPLICATE_KEY; nor AGENT_NOT_AVAILABLE, TIMEOUT, or RESUME_WINDOW_EXPIRED from §12.

Proposed fix

Add #[serde(rename = "DUPLICATE_KEY")] DuplicateKey (and, while there, AgentNotAvailable → "AGENT_NOT_AVAILABLE") to ErrorCode, wire as_str/round-trip tests, and change the conflict arm in spawn_tool_invoke to use ErrorCode::DuplicateKey with retryable: Some(false). Update the conformance test to assert the wire code.

Acceptance criteria

• A second tool.invoke with the same idempotency key but different arguments yields an error carrying wire code DUPLICATE_KEY.
• ErrorCode serde round-trips DUPLICATE_KEY.