Fixup a bug in the sync polling exec function (#29)

In the case where the response had to poll, there was a silly mistake
where we were reading the wrong field off the response.

Previously:
```julia
julia> RAI.exec(ctx, "nhd-test-1", "nhd-s", """
           2 + 2 * 2, "hi", 'a'
           """)
ERROR: KeyError: key "id" not found
Stacktrace:
 [1] getindex(h::Dict{String, JSON3.Object{Vector{UInt8}, Vector{UInt64}}}, key::String)
   @ Base ./dict.jl:481
 [2] exec(ctx::Context, database::String, engine::String, source::String; inputs::Nothing, readonly::Bool, kw::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ RAI ~/work/rai-sdk-julia/src/api.jl:403
```

This is now fixed:
```julia
julia> RAI.exec(ctx, "nhd-test-1", "nhd-s", """
           2 + 2 * 2, "hi", 'a', "hello", '2', 2.0
           """)
Dict{String, Any} with 4 entries:
  "metadata"    => JSON3.Object[{…
  "problems"    => Union{}[]
  "results"     => Pair{String, Arrow.Table}["/:output/Int64/String/…
  "transaction" => {…
```

Author: NHDaly

src/api.jl

@@ -400,13 +400,13 @@ function exec(ctx::Context, database::AbstractString, engine::AbstractString, so
         for duration in backoff
             transaction_is_done(txn) && break
 
-            txn = get_transaction(ctx, txn["id"])
+            txn = get_transaction(ctx, transaction_id(txn))
             sleep(duration)
         end
         if haskey(txn, "results")
             return txn
         else
-            id = txn["id"]
+            id = transaction_id(txn)
             t = @spawn get_transaction(ctx, id)
             m = @spawn get_transaction_metadata(ctx, id)
             p = @spawn get_transaction_problems(ctx, id)
@@ -419,9 +419,10 @@ function exec(ctx::Context, database::AbstractString, engine::AbstractString, so
             )
         end
     catch
+        @info "TXN" txn
         # Always print out the transaction id so that users can still get the txn ID even
         # if there's an error during polling (such as an InterruptException).
-        @info """Exception while polling for transaction:\n"id": $(repr(txn["id"]))"""
+        #@info """Exception while polling for transaction:\n"id": $(repr(transaction_id(txn)))"""
         rethrow()
     end
 end
@@ -474,6 +475,13 @@ function transaction_is_done(txn)
     return txn["state"] ∈ ("COMPLETED", "ABORTED")
 end
 
+function transaction_id(txn)
+    if haskey(txn, "transaction")
+        txn = txn["transaction"]
+    end
+    return txn["id"]
+end
+
 function get_transaction_metadata(ctx::Context, id::AbstractString; kw...)
     path = PATH_ASYNC_TRANSACTIONS * "/$id/metadata"
     rsp = _get(ctx, path; kw...)