fix: bridge-fee arithmetic overflow remediation

[Dopezapha]

Closes #306

• Description:
  This PR addresses a critical arithmetic safety issue in the cross-chain token bridge (token-bridge contract) where bridge fee calculations used unchecked multiplication.

  Key Changes:

  1. Safety Hardening: The bridge fee calculation in deposit_for_bridge now uses checked_mul. This prevents potential intermediate result overflows when handling extremely large transfer amounts
  2. Explicit Failure (Production Grade): Implemented an explicit .expect() to ensure the contract halts safely with a descriptive error message if an overflow is reached, rather than silently wrapping to an incorrect value.
  3. Comprehensive Testing: Added specific test scenarios to the bridge's local test suite that trigger the overflow case and confirm correct normal operation.

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