fix(perps): preserve integer trailing zeros when szDecimals=0 (#29016)

## **Description**

`formatPositionSize` in `app/controllers/perps/utils/perpsFormatters.ts`
uses

```ts
value.toFixed(szDecimals).replace(/\.?0+$/u, '')
```

to strip trailing zeros. Because the decimal point is optional in the
regex, the pattern also matches trailing zeros on pure integer strings —
so whenever `szDecimals = 0`, valid zeros are eaten:

- `formatPositionSize(100, 0)` → `"1"`   (expected `"100"`)
- `formatPositionSize(20, 0)`  → `"2"`   (expected `"20"`)
- `formatPositionSize(1000, 0)` → `"1"`  (expected `"1000"`)

This affects every HyperLiquid asset with `szDecimals = 0` (whole-unit
assets). The magnitude-based fallback path below is unaffected — it
always calls `toFixed(2/4/6)`, so a decimal point is guaranteed.

### Fix

Only strip trailing zeros when `toFixed` actually produced a decimal
point:

```ts
const fixed = value.toFixed(szDecimals);
return fixed.includes('.') ? fixed.replace(/\.?0+$/u, '') : fixed;
```

### Cross-reference

Same bug was caught by Cursor Bugbot on the extension copy and fixed in
[metamask-extension#41920](MetaMask/metamask-extension#41920)
(commit `ed9de1f1`). The extension currently mirrors this file verbatim,
so mobile needs the same fix to stay authoritative.

## **Changelog**

CHANGELOG entry: Fixed a perps position size formatting bug that
stripped valid trailing zeros on whole-unit assets (szDecimals=0), e.g.
displaying "1" instead of "100".

## **Related issues**

Fixes:
[TAT-2992](https://consensyssoftware.atlassian.net/browse/TAT-2992)

## **Manual testing steps**

```gherkin
Feature: Position size formatting for whole-unit assets

  Scenario: user views a position on a HyperLiquid asset with szDecimals=0
    Given the asset has szDecimals=0 in HyperLiquid metadata
    And the user holds a position of 100 units

    When the position size is rendered in the perps UI
    Then the displayed size is "100" (not "1")
```

Unit tests cover the regression directly (see new cases added to
`perpsFormatters.test.ts`).

## **Screenshots/Recordings**

### **Before**

n/a — unit-test-only fix; reproduced via `formatPositionSize(100, 0) ===
"1"`.

### **After**

n/a — `formatPositionSize(100, 0) === "100"` (new test cases).

## **Pre-merge author checklist**

- [x] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask Mobile
Coding
Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [x] I've completed the PR template to the best of my ability
- [x] I've included tests if applicable
- [x] I've documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [x] I've applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

#### Performance checks (if applicable)

Not applicable — pure string-formatting fix with no perf impact.

- [ ] I've tested on Android
- [ ] I've tested with a power user scenario
- [ ] I've instrumented key operations with Sentry traces for production
performance metrics

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.


[TAT-2992]:
https://consensyssoftware.atlassian.net/browse/TAT-2992?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk, isolated change to position-size string formatting that only
affects the `szDecimals` path (especially `szDecimals=0`) and is covered
by a targeted regression test.
> 
> **Overview**
> Fixes `formatPositionSize` so whole-unit assets with `szDecimals=0` no
longer lose significant trailing zeros (e.g., `"100"` incorrectly
becoming `"1"`). The formatter now only strips trailing zeros when
`toFixed(szDecimals)` produces a decimal point, and adds a regression
test suite covering integer and rounding behavior for `szDecimals=0`.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
e732c53. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: abretonc7s

app/controllers/perps/utils/perpsFormatters.test.ts

@@ -178,6 +178,14 @@ describe('formatPositionSize', () => {
     expect(formatPositionSize(44.0, 2)).toBe('44');
   });
 
+  it('preserves integer trailing zeros when szDecimals=0 (whole-unit assets)', () => {
+    expect(formatPositionSize(100, 0)).toBe('100');
+    expect(formatPositionSize(20, 0)).toBe('20');
+    expect(formatPositionSize(1000, 0)).toBe('1000');
+    expect(formatPositionSize(1, 0)).toBe('1');
+    expect(formatPositionSize(1.7, 0)).toBe('2');
+  });
+
   it('uses magnitude logic for very small values (< 0.01) without szDecimals', () => {
     const result = formatPositionSize(0.00009);
     expect(result).toBe('0.00009');

app/controllers/perps/utils/perpsFormatters.ts

@@ -522,7 +522,11 @@ export const formatPositionSize = (
 
   // Use asset-specific decimals if provided (Hyperliquid metadata)
   if (szDecimals !== undefined) {
-    return value.toFixed(szDecimals).replace(/\.?0+$/u, '');
+    const fixed = value.toFixed(szDecimals);
+    // Only strip trailing zeros when a decimal point is present; toFixed(0)
+    // returns an integer string and the regex would otherwise eat valid zeros
+    // on whole-unit assets (szDecimals=0), e.g. "100" -> "1".
+    return fixed.includes('.') ? fixed.replace(/\.?0+$/u, '') : fixed;
   }
 
   // Fallback: magnitude-based decimal logic for backwards compatibility