Simplify SchemaId generation to use GetHashCode

Agent-Logs-Url: https://github.com/fsprojects/FSharp.Data.GraphQL/sessions/70eb1609-2aab-4948-8d69-52c900443420

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

Author: Copilot

src/FSharp.Data.GraphQL.Shared/ValidationResultCache.fs

@@ -3,12 +3,6 @@ namespace FSharp.Data.GraphQL.Validation
 open FSharp.Data.GraphQL
 open FSharp.Data.GraphQL.Types.Introspection
 open System
-open System.IO
-open System.Security.Cryptography
-open System.Text
-open System.Text.Encodings.Web
-open System.Text.Json
-open System.Text.Json.Serialization
 
 type ValidationResultKey =
     { DocumentId : string
@@ -21,35 +15,14 @@ type IValidationResultCache =
     abstract GetOrAdd : ValidationResultProducer -> ValidationResultKey -> ValidationResult<GQLProblemDetails>
 
 module SchemaId =
-
-    let private formatByteAsLowerHex (value : byte) =
-        value.ToString("x2", System.Globalization.CultureInfo.InvariantCulture)
-    
-    // Note: UnsafeRelaxedJsonEscaping is used here only for deterministic hashing,
-    // not for output to untrusted contexts. The JSON is never exposed externally.
-    let private jsonOptions = JsonSerializerOptions(
-        WriteIndented = false,
-        DefaultIgnoreCondition = JsonIgnoreCondition.Never,
-        PropertyNamingPolicy = null,
-        Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping
-    )
-    
     /// <summary>
-    /// Computes a deterministic schema identifier from an introspection schema.
+    /// Computes an in-memory schema identifier from an introspection schema instance.
     /// </summary>
-    /// <param name="introspectionSchema">The introspection schema to hash.</param>
-    /// <returns>A lowercase hexadecimal SHA-256 hash string that uniquely identifies the schema structure.</returns>
+    /// <param name="introspectionSchema">The introspection schema.</param>
+    /// <returns>A string representation of the schema hash code.</returns>
     [<CompiledName("FromIntrospectionSchema")>]
     let fromIntrospectionSchema (introspectionSchema : IntrospectionSchema) =
-        use stream = new MemoryStream()
-        JsonSerializer.Serialize(stream, introspectionSchema, jsonOptions)
-        // Note: Creating SHA256 instance per call is acceptable since schema ID computation
-        // happens infrequently (typically once per schema during validation cache key creation)
-        use sha256 = SHA256.Create()
-        let hash = sha256.ComputeHash(stream.ToArray())
-        hash
-        |> Seq.map formatByteAsLowerHex
-        |> String.concat ""
+        introspectionSchema.GetHashCode().ToString(System.Globalization.CultureInfo.InvariantCulture)
 
 /// An in-memory cache for the results of schema/document validations, with a lifetime of 30 seconds.
 type MemoryValidationResultCache () =