fix(token-swap): correct ratio calculation and liquidity assertion in deposit_liquidity

[HkSolDev]

Summary

Fixes the deposit_liquidity instruction which had incorrect ratio
calculation logic and test assertions that didn't catch the bug.

Root Cause

The ratio was computed with checked_mul (pool_a × pool_b) instead of
checked_div (pool_a / pool_b), so deposit proportionality was wrong.
Tests were also passing the wrong token amount for the B slot, masking
the bug entirely.

Changes

programs/token-swap/src/instructions/deposit_liquidity.rs

• Changed ratio from pool_a.checked_mul(pool_b) → pool_a.checked_div(pool_b)
  so proportionality is calculated correctly
• Replaced .unwrap() with .ok_or(DepositTooSmall)? on checked_div
  to avoid panic when pool_b is zero

tests/deposit-liquidity.ts

• Pass depositAmountB (1M) instead of depositAmountA (4M) for token B slot
  so unequal deposits are actually tested
• Fix liquidity assertion: use sqrt(A × B) - minLiquidity formula
  instead of A - minLiquidity which only worked when A == B
• Fix token B balance assertion: subtract depositAmountB not depositAmountA

Testing

```bash
cargo build-sbf # ✅ passes (warnings only)
anchor test # ✅ 7/7 tests pass
```

Closes #512

[HkSolDev]

@claude