fix: add typed contract revert errors

[silentgeckoaudit3801]

Closes #148.

Summary

• add ContractRevertError with reason, rawData, rpcCode, and optional decoded custom error metadata
• re-export the typed transaction errors from errors/index.ts
• make RpcProvider throw ContractRevertError for JSON-RPC contract reverts instead of a generic WhiteChainError
• cover instanceof discrimination and revert metadata in focused tests

Validation

• git diff --check
• rg -n "ContractRevertError|extractRevertReason|isContractRevertError|rawData|rpcCode" src tests
• attempted vitest run tests/providers/RpcProvider.test.ts tests/errors.test.ts, but the existing checkout's node_modules is missing Rollup's optional native package @rollup/rollup-win32-x64-msvc; I did not install dependencies