fix FixedArray::get and corresponding doc test

[myfreess]

No description provided.

[peter-jerry-ye-code-review]

Example tests in documentation don't match the updated function signature

Category
Correctness
Code Snippet
test "FixedArray::get" {
let arr : FixedArray[Int] = [1, 2, 3]
inspect!(arr.get(1), content="Some(2)")
}
Recommendation
Update example to explicitly handle the optional return type:

test "FixedArray::get" {
  let arr : FixedArray[Int] = [1, 2, 3]
  match arr.get(1) {
    Some(value) => inspect!(value, content="2")
    None => fail!("unexpected None")
  }
}```
**Reasoning**
The current example might be misleading as it doesn't demonstrate proper pattern matching on optional values, which is important for the new API

</details>
<details>

<summary> Guard condition could be more readable with a single boundary check </summary>

**Category**
Maintainability
**Code Snippet**
guard idx >= 0 && idx < len else { None }
**Recommendation**
Consider introducing a helper function:
```moonbit
fn is_valid_index(idx: Int, len: Int) -> Bool {
  idx >= 0 && idx < len
}

// Then use:
guard is_valid_index(idx, len) else { None }```
**Reasoning**
A named helper function would make the intent clearer and could be reused in other array operations that need bounds checking

</details>
<details>

<summary> Consider adding test case for negative indices </summary>

**Category**
Correctness
**Code Snippet**
test "FixedArray::get/out_of_bounds" {
  let arr : FixedArray[Int] = [1, 2, 3]
  inspect!(arr.get(3), content="None")
}
**Recommendation**
Add test case for negative indices:
```moonbit
test "FixedArray::get/negative_index" {
  let arr : FixedArray[Int] = [1, 2, 3]
  inspect!(arr.get(-1), content="None")
}```
**Reasoning**
The current test suite only verifies upper bound checking. Testing negative indices would ensure complete bounds checking behavior

</details>

[coveralls]

Pull Request Test Coverage Report for Build 6878

Details

• 0 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 92.421%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
builtin/fixedarray.mbt	0	3	0.0%

Totals
Change from base Build 6872:	-0.03%
Covered Lines:	8695
Relevant Lines:	9408

---

💛 - Coveralls

[peter-jerry-ye]

Requires migration phase