perf(l1): remove duplicate checks in static_left_pad

[MegaRedHand]

Motivation

static_left_pad appears in some flamegraphs, due to being used to encode bytecode metadata. This PR improves a bit the implementation.

Description

This PR removes some duplicate checks from the implementation, which might improve performance.

[Copilot]

Pull Request Overview

This PR optimizes the static_left_pad function used for encoding bytecode metadata by eliminating redundant bounds checking operations.

Key Changes:

• Replaces explicit length check + saturating_sub + get_mut().ok_or() with a single checked_sub operation
• Removes redundant bounds check by using direct indexing instead of get_mut()

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[github-actions]

Lines of code report

Total lines added: 0
Total lines removed: 4
Total lines changed: 4

Detailed view

+------------------------------------+-------+------+
| File                               | Lines | Diff |
+------------------------------------+-------+------+
| ethrex/crates/common/rlp/decode.rs | 649   | -4   |
+------------------------------------+-------+------+

[github-actions]

Benchmark Block Execution Results Comparison Against Main

Command	Mean [s]	Min [s]	Max [s]	Relative
base	60.714 ± 0.342	60.317	61.170	1.00
head	61.197 ± 0.660	60.539	62.823	1.01 ± 0.01