Function resolve_explicit_namespace 

pub fn resolve_explicit_namespace(
    params: &Value,
    default_namespace: &str,
) -> Result<Namespace, RuntimeError>

Resolve and validate a caller-supplied namespace argument the same way on every MCP ingress path.

• Absent namespace key → parse default_namespace.
• Present namespace: "<string>" → parse the caller’s value.
• Present non-string namespace (null, number, bool, array, object) → fail closed with RuntimeError::InvalidInput. ADR-018 requires this: a malformed explicit value must never be silently coerced to the default namespace.

This is the single chokepoint both VerbRegistry::dispatch (single-backend and JSON-form ingress) and the multi-backend coordinator intercept (dispatch_via_coordinator_inner in khive-mcp) call into, so no ingress path can bypass the fail-closed rule by routing around dispatch.