fix(handler): use cfg gas_params in validate_initial_tx_gas

[bluebluesaku]

validate_initial_tx_gas was constructing a fresh GasParams::new_spec(spec) instead of reading from cfg.gas_params(), ignoring custom gas parameter overrides. This causes validation to reject valid transactions on custom chains with modified gas schedules.

[codspeed-hq]

Merging this PR will not alter performance

✅ 176 untouched benchmarks

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_{Comparing bluebluesaku:cfg (c7059d5) with main (e03804e)}