Handle Generic Context Arg in runtime implementation of signature key computation (#128171)

> [!NOTE]
> PR description generated with AI assistance.

Fixes issue where methods with generic instantiation arguments cannot be
called from managed code if they are called from R2R code.

The WASM signature key computation in `helpers.cpp` was not accounting
for `HasGenericContextArg()`, which meant that methods requiring a
generic instantiation context (hidden argument) would produce an
incorrect signature key. This caused call mismatches when managed code
invoked R2R-compiled methods with generic context.

The fix adds a check for `HasGenericContextArg()` and appends the
appropriate `'i'` (integer/pointer) parameter to the signature key
buffer, matching the actual calling convention.

Author: davidwrighton

src/coreclr/vm/wasm/helpers.cpp

@@ -947,6 +947,13 @@ namespace
             pos++;
         }
 
+        if (sig.HasGenericContextArg())
+        {
+            if (pos < maxSize)
+                keyBuffer[pos] = 'i';
+            pos++;
+        }
+
         for (CorElementType argType = sig.NextArg();
             argType != ELEMENT_TYPE_END;
             argType = sig.NextArg())