Enforce EIP-7825 per-tx gas cap on settlement

[0xDevNinja]

Description

The driver caps a settlement's gas estimate at half the block gas limit (e.g. 60M on a 120M block). Fusaka introduced the EIP-7825 per-transaction gas cap of 2^24 - 1 = 16,777,215; any tx exceeding this is rejected by the mempool. Without this check a solution above the cap can still be returned by /solve and forwarded to /settle, where it could never be mined. The cap is already enforced on the quote-verification path (#4261) but was never ported to settlement submission.

Changes

• In Gas::new, cap the per-settlement maximum at min(block_limit / 2, EIP-7825) so over-sized solutions fail fast through the existing GasLimitExceeded error.
• Expose the cap as Gas::EIP_7825_TX_GAS_CAP so callers and tests share the constant.
• Chains with a block gas limit below 2 * EIP-7825 keep the tighter half-block behaviour.
• Add unit tests for the new cap, the boundary case at the cap, and the small-block-limit path.

How to test

cargo nextest run -p driver settlement::tests

All three new tests pass; existing driver tests are unchanged.

Fixes #4368

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[gemini-code-assist]

Code Review

This pull request implements the EIP-7825 per-transaction gas cap (2^24 - 1) within the settlement domain. The Gas::new function now caps the maximum gas by taking the minimum of half the block limit and this new constant, preventing the creation of solutions that would be rejected by the mempool. Unit tests have been added to verify the cap's behavior in both large and small block limit scenarios. I have no feedback to provide.