Switch gas report parsing from RST to native JSON output

[r0qs]

Currently, our external tests use hardhat-gas-reporter with RST table output, which we then parse with parse_eth_gas_report.py to convert to JSON. This approach is fragile:

1. RST parsing requires complex regex patterns and is an unnecessary intermediate step done when hardhat-gas-reporter didn't support JSON output
2. Recent updates to hardhat-gas-reporter v2 already required adjustments to handle number formatting changes

Since hardhat-gas-reporter v2, native JSON output is supported via the outputJSON option. We should switch to it.