fix(predict): round fee decimals to avoid underflow errors

[matallui]

Description

Fixed a floating-point precision issue in the Polymarket fee calculation that was causing order placement failures. When calculating fees (e.g., for a $7.40 bet), JavaScript floating-point arithmetic would produce values with excessive decimal places (e.g., 0.29600000000000004), which would later fail when passed to parseUnits. The fix rounds the fee to 4 decimal places to prevent these precision errors while maintaining sufficient accuracy for fee calculations.

Changelog

CHANGELOG entry: null

Related issues

Fixes:

Manual testing steps

Feature: Polymarket bet placement

  Scenario: user places a bet with amount that triggers floating-point precision issue
    Given user is on the Predict feature
    And user has selected a Polymarket prediction market

    When user enters a bet amount of $7.40
    And user confirms the bet placement
    Then the order should be successfully placed without errors

Screenshots/Recordings

Before

Order placement would fail with parseUnits error when fee calculation resulted in values like 0.29600000000000004

After

Order placement succeeds as fee is properly rounded to 0.2960 (4 decimal places)

Pre-merge author checklist

• I've followed MetaMask Contributor Docs and MetaMask Mobile Coding Standards.
• I've completed the PR template to the best of my ability
• I've included tests if applicable
• I've documented my code using JSDoc format if applicable
• I've applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

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.

---

Note

Round totalFee to 4 decimals in calculateFees to prevent floating-point precision issues.

^{Written by Cursor Bugbot for commit b198cda. This will update automatically on new commits. Configure here.}

[github-actions]

CLA Signature Action: All authors have signed the CLA. You may need to manually re-run the blocking PR check if it doesn't pass in a few minutes.

[sonarqubecloud]

Quality Gate failed

Failed conditions
E Maintainability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

Catch issues before they fail your Quality Gate with our IDE extension SonarQube for IDE

[caieu]

LGTM