fix(evm): reject malformed CAIP-2 network identifiers in getEvmChainId - #3521

Description

getEvmChainId parses the CAIP-2 reference with parseInt(idStr, 10), which stops at the first non-digit instead of rejecting the input, so a malformed network resolves to a chain ID the caller never supplied:

NetworkSchemaV2 only requires a colon, so these reach the mechanism. The value is then signed into the EIP-712 domain — getBatchSettlementEip712Domain(getEvmChainId(network)) in batch-settlement/server/verify.ts and batch-settlement/utils.ts, and the Permit2 / EIP-3009 domains via shared/permit2.ts — and selects the RPC in shared/rpc.ts.

The fix requires a bare decimal reference. /^eip155:(\d+)$/ is the same pattern the SIWX extension already uses (packages/extensions/src/sign-in-with-x/evm.ts). Chain IDs past Number.MAX_SAFE_INTEGER now throw instead of rounding, since number cannot represent what Go's big.Int and Python's int can.

Well-formed identifiers are unaffected and both existing error messages are unchanged.

Tests

• pnpm --filter @x402/evm test: 40 files, 1129 passed (1121 on main, +8 new).
• The 8 new cases fail on main (AssertionError: expected [Function] to throw an error) and pass with the fix.
• eslint and prettier --check clean for the package.

Not run locally: the full pnpm test matrix and the Go/Python suites (unchanged here) — CI covers those. I also grepped every "eip155:…" literal under packages/; the only ones the stricter pattern rejects are negative-test fixtures.

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge)
• I added a changelog fragment for user-facing changes

AI-assisted, per AI-Assisted Contributions: the patch was produced with Claude. The diff, the before/after behaviour table and the local test runs were reviewed before marking this ready.