Decode revert data against the ABI: custom errors, Error(string), and Panic(uint256)

[koko1123]

Motivation

Provider now captures the raw revert payload -- RpcErrorInfo carries the JSON-RPC code and the error.data hex (truncated to 512 bytes) -- but the caller still gets a hex string. Every modern Solidity contract reverts with a custom error (error InsufficientBalance(uint256 available, uint256 required)), so "why did my call fail" is currently answered with 0x556f1830... and a manual trip to a 4-byte database.

The two halves needed to close this are both already in the tree and simply not wired together: abi_json.zig parses "type": "error" entries into abi_types.AbiError, and abi_types.selectorFromSignature computes the 4-byte selector. This is a small, high-visibility quality-of-life win and a good first issue.

Scope

• decodeRevert(allocator, revert_data, abi) !DecodedRevert in a new src/revert.zig (or alongside abi_decode), matching revert_data[0..4] against the selector of each AbiError in the parsed ABI and decoding the remainder with the existing abi_decode machinery
• Handle the three standard shapes before any custom error:
  • Error(string) -- selector 0x08c379a0, the classic require(cond, "message")
  • Panic(uint256) -- selector 0x4e487b71, with the panic codes mapped to names (0x01 assert, 0x11 arithmetic overflow, 0x12 division by zero, 0x32 array out of bounds, and the rest)
  • empty revert data -- a bare revert() or an out-of-gas, which must be reported as "no reason given" rather than as a decode failure
• DecodedRevert as a tagged union over { string_error, panic, custom, unknown }, where unknown preserves the raw bytes and the unmatched selector so the caller can still log something useful
• A comptime twin on the abigen bindings: Contract.decodeRevert(revert_data) resolving against that contract's own errors, matching how decodeEvent already works there
• Note the 512-byte truncation in RpcErrorInfo: a revert carrying a long string or a dynamic array can be cut off. Either raise the cap for the decode path or return a clearly-flagged partial decode -- silently decoding truncated data into wrong values would be worse than not decoding at all.

Testing

Entirely offline. Encode known reverts by hand and assert the round trip: a require string, each panic code, a custom error with static params, a custom error with a dynamic param, an unknown selector, and empty data. abi_json.zig already has an InsufficientBalance error fixture in its tests to build on.

Pointers

src/provider.zig (the RpcErrorInfo capture path and its truncation comment near line 42), src/abi_json.zig:53 (error-entry parsing) and src/abi_types.zig:307 (AbiError), src/abi_types.zig:316 (selectorFromSignature), src/abi_decode.zig for the parameter decoding, and src/abigen.zig:329 (decodeEvent) as the template for the comptime binding.