fix: avoid integer overflow in MB→bytes conversion

[kilavvy]

Description:

• Summary: Replace MB-to-bytes multiplications with saturating_mul and introduce MB_TO_BYTES constant in mempool.rs.
• Why: Prevent silent u64 wrap-around in release builds that could shrink mempool size limits unexpectedly.
• Changes:
  • Use: saturating_mul for pending/basefee/queued size limits.
  • Add: MB_TO_BYTES constant for clarity.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.5%. Comparing base (0f617e2) to head (cc21f4b).

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/sequencer/src/mempool.rs	74.1% <100.0%> (ø)

... and 4 files with indirect coverage changes

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