Switch gas report parsing from RST to native JSON output

Currently, our external tests use `hardhat-gas-reporter` with RST table output, which we then parse with [parse_eth_gas_report.py](https://github.com/ethereum/solidity/blob/v0.8.33/scripts/externalTests/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.

Start with scripts/externalTests/parse_eth_gas_report.py, then trace the external-test setup that invokes hardhat-gas-reporter and consumes its output. Run the affected external tests to establish the current behavior. Done means the tests use native JSON output, no longer depend on RST parsing, and still produce the expected gas data.

Written by the indexing model from the issue text.