Refine hash span usage and test value explanation

Agent-Logs-Url: https://github.com/fsprojects/FSharp.Data.GraphQL/sessions/020d0296-28b1-416d-bd12-9dd0790516ba

Co-authored-by: xperiandri <2365592+xperiandri@users.noreply.github.com>

Author: Copilot

src/FSharp.Data.GraphQL.Server/Executor.fs

@@ -88,7 +88,7 @@ type Executor<'Root>(schema: ISchema<'Root>, middlewares : IExecutorMiddleware s
         let canonicalQuery = document.ToQueryString()
         let queryBytes = Encoding.UTF8.GetBytes canonicalQuery
         let hash = SHA256.HashData queryBytes
-        BinaryPrimitives.ReadInt32BigEndian(ReadOnlySpan<byte>(hash, 0, 4))
+        BinaryPrimitives.ReadInt32BigEndian(hash.AsSpan(0, 4))
 
     let rec runMiddlewares (phaseSel : IExecutorMiddleware -> ('ctx -> ('ctx -> 'res) -> 'res) option)
                            (initialCtx : 'ctx)

tests/FSharp.Data.GraphQL.Tests/ExecutionTests.fs

@@ -386,6 +386,7 @@ let ``Execution when querying returns unique document id with response`` () =
     let query = "query Example { a, b, a }"
     // Deterministic SHA-256-based documentId for canonical `query Example { a b a }`,
     // using the first 4 bytes of the hash as a big-endian int32.
+    // Computed once via parse + ToQueryString + SHA-256 and kept fixed to catch regressions.
     let expectedDocumentId = -2063861555
     let result1 = sync <| Executor(schema).AsyncExecute(query, getMockInputContext, { A = "aa"; B = 2 })
     let result2 = sync <| Executor(schema).AsyncExecute(query, getMockInputContext, { A = "aa"; B = 2 })