test: improve coverage for strconv/double.mbt

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

## Summary

```
coverage of `strconv/double.mbt`: 74.1% -> 81.5%
```

strconv/double_test.mbt

@@ -0,0 +1,38 @@
+// 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.
+
+test "try_fast_path overflow when shift is too large" {
+  // When the shift (exponent - max_exponent_fast_path) is too large,
+  // the multiplication of mantissa with int_pow10[shift] will overflow,
+  // triggering line 130
+  let result = try {
+    @strconv.parse_double!("9007199254740992e30")
+  } catch {
+    _ => panic()
+  }
+  // The function successfully falls back to slow path
+  inspect!(result, content="9.007199254740992e+45")
+}
+
+test "try_fast_path overflow when mantissa is too large" {
+  // When the mantissa after shifting is larger than max_mantissa_fast_path,
+  // line 133 will be triggered
+  let result = try {
+    @strconv.parse_double!("9007199254740992e23")
+  } catch {
+    _ => panic()
+  }
+  // The function successfully falls back to slow path
+  inspect!(result, content="9.007199254740991e+38")
+}