fix(tests): clamp reserve balance threshold at zero

[pdobacz]

MERGE after #42.

Add a spec test that a sender whose gas fees exceed the reserve can empty.

Also throw a meaningful assertion error on txs impossible to come from consensus, when gas fees exceed reserve but sender cannot empty.

Both were missed in the original reserve balance implementation (in execution-specs that is), and the implementation would just throw an OverflowError in both cases (U256 underflow to be precise)

Greptile Summary

The PR clamps the sender reserve threshold at zero when gas fees equal or exceed the reserve and adds an assertion for transactions that consensus should not sequence.

• Applies the reserve-threshold correction consistently across Monad Eight, Nine, Ten, and Next.
• Adds boundary coverage for gas fees below, above, and around the reserve.

Confidence Score: 5/5

The PR appears safe to merge with no actionable correctness or security issues identified.

The clamp prevents unsigned subtraction underflow for senders allowed to empty their balance, while the assertion preserves the stated consensus restriction for other senders and the new tests cover the relevant fee boundaries.

Important Files Changed

Filename	Overview
src/ethereum/forks/monad_eight/vm/interpreter.py	Clamps the sender threshold before reserve validation and explicitly rejects consensus-impossible non-exception transactions.
src/ethereum/forks/monad_nine/vm/interpreter.py	Carries the corrected reserve-threshold calculation and assertion into Monad Nine.
src/ethereum/forks/monad_ten/vm/interpreter.py	Carries the corrected reserve-threshold calculation and assertion into Monad Ten.
src/ethereum/forks/monad_next/vm/interpreter.py	Carries the corrected reserve-threshold calculation and assertion into Monad Next.
tests/monad_eight/reserve_balance/test_gas_fees_vs_reserve.py	Adds regression coverage spanning gas fees below and above the reserve, including the former unsigned-underflow boundary.

_{Reviews (1): Last reviewed commit: "fix(spec-specs, tests): clamp reserve ba..." | Re-trigger Greptile}

[QEDK]

LGTM.