test: add Rex1 compute gas detention reset tests

[pieakshat]

Summary

Adds a dedicated tests/rex1/ module covering the Rex1 fix for compute gas detention state leaking between transactions.

Before Rex1, volatile data access (oracle call, TIMESTAMP, etc.) in TX1 would lower the detained compute gas limit, and that lowered limit persisted
into subsequent transactions within the same block — causing unrelated transactions to fail with ComputeGasLimitExceeded.
Rex1 fixes this by resetting detained_limit to the base TX compute gas limit at the start of each transaction.

Five tests are added across three scenarios:

• Oracle detention (CALL to oracle contract): Rex1 resets the 1M cap between txs; REX spec does not (bug preserved for backward compat).
• Block-env detention (TIMESTAMP opcode): same reset behavior for the 20M block-env cap — this scenario had no existing test coverage.
• Multi-transaction reset: verifies the reset is persistent across multiple rounds of volatile → expensive transactions, not a one-time event.

Test plan

• cargo test -p mega-evm --test rex1 — all 5 tests pass.
• cargo test -p mega-evm — full suite passes with no regressions.

Labels checklist

• spec: spec:stable
• comp: comp:core
• api: api:unchanged

[pieakshat]

hi, any updates on this?