feat: add TestCoin helper class

[airope]

Summary

Adds a TestCoin helper class, similar to the existing TestMessage class, for creating coins in tests.

Closes #3445

Changes

• packages/account/src/test-utils/test-coin.ts — New TestCoin class with:
  • Constructor with sensible defaults (random owner, 1M amount, base asset ID)
  • toChainCoin() method to convert to chain snapshot format
  • Static many() method to generate multiple test coins at once
• packages/account/src/test-utils/test-coin.test.ts — Unit tests covering defaults, custom values, owner override, and many()
• packages/account/src/test-utils.ts — Re-export TestCoin

Usage

import { TestCoin } from @fuel-ts/account/test-utils;

// Single coin with defaults
const coin = new TestCoin();

// Multiple coins for a specific owner
const coins = TestCoin.many({ owner: myAddress }, 4);

// Convert to chain format for snapshot configs
const chainCoin = coin.toChainCoin();

[cursor]

PR Summary

Low Risk
Low risk: adds a new test-only helper and unit tests, with minimal surface-area change beyond a new export.

Overview
Adds a new TestCoin test utility for building coin entries used in genesis/snapshot configs, including defaults, custom overrides, and a many() factory plus toChainCoin() conversion.

Exports TestCoin from packages/account/src/test-utils.ts and adds a dedicated test suite validating default fields, overrides (including owner override), and multi-coin generation behavior.

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