test: improve coverage for builtin/tuple_hash.mbt

**Disclaimer:** This PR was generated by an LLM agent as part of an experiment.

## Summary

```
coverage of builtin/tuple_hash.mbt: 0% -> 100%
```

Author: rami3l

builtin/tuple_hash_test.mbt

@@ -0,0 +1,90 @@
+// Copyright 2025 International Digital Economy Academy
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+///|
+let seed = 0x533D
+
+///|
+fn h() -> Hasher {
+  Hasher::new(seed~)
+}
+
+///|
+test "hash_combine for 2-tuple" {
+  assert_eq!(
+    h()..combine(1)..combine("hello").finalize(),
+    h()..combine((1, "hello")).finalize(),
+  )
+}
+
+///|
+test "hash_combine for 3-tuple" {
+  assert_eq!(
+    h()..combine(1)..combine("hello")..combine(3.14).finalize(),
+    h()..combine((1, "hello", 3.14)).finalize(),
+  )
+}
+
+///|
+test "hash_combine for 4-tuple" {
+  assert_eq!(
+    h()..combine(1)..combine("hello")..combine(3.14)..combine(15).finalize(),
+    h()..combine((1, "hello", 3.14, 15)).finalize(),
+  )
+}
+
+///|
+test "hash_combine for 5-tuple" {
+  assert_eq!(
+    h()
+    ..combine(1)
+    ..combine("hello")
+    ..combine(3.14)
+    ..combine(15)
+    ..combine('a')
+    .finalize(),
+    h()..combine((1, "hello", 3.14, 15, 'a')).finalize(),
+  )
+}
+
+///|
+test "hash_combine for 6-tuple" {
+  assert_eq!(
+    h()
+    ..combine(1)
+    ..combine("hello")
+    ..combine(3.14)
+    ..combine(15)
+    ..combine('a')
+    ..combine(0x12345678UL)
+    .finalize(),
+    h()..combine((1, "hello", 3.14, 15, 'a', 0x12345678UL)).finalize(),
+  )
+}
+
+///|
+test "hash_combine for 7-tuple" {
+  assert_eq!(
+    h()
+    ..combine(1)
+    ..combine("hello")
+    ..combine(3.14)
+    ..combine(15)
+    ..combine('a')
+    ..combine(0x12345678UL)
+    ..combine(b's')
+    .finalize(),
+    h()..combine((1, "hello", 3.14, 15, 'a', 0x12345678UL, b's')).finalize(),
+  )
+}