fix: subscription use bignumber.js for precise crypto approval amount calculation

[tuna1207]

Explanation

Fix subscription crypto approval amount calculation by using bignumber.js

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed, highlighting breaking changes as necessary
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes

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Note

Switches subscription price and token approval calculations to bignumber.js for precision; adds dependency and updates changelog.

• Subscription Controller:
  • Replace float/BigInt math with bignumber.js in src/SubscriptionController.ts:
    • #getSubscriptionPriceAmount now uses BigNumber for price computation.
    • getTokenApproveAmount now uses BigNumber and returns toFixed(0).
• Dependencies:
  • Add bignumber.js to packages/subscription-controller/package.json and yarn.lock.
• Changelog:
  • Note precision fix for getTokenApproveAmount using bignumber.js in CHANGELOG.md.

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