fix(viem): omit missing typed-data chainId

[omerbek]

Summary

• avoid converting a missing typed-data domain.chainId into NaN in the viem signer adapter
• pass undefined when chainId is absent, matching the optional handling used by the viem seaport bridge
• add a regression test for signing typed data with no chainId in the domain

Fixes #1994.

Testing

• npm run test -- test/provider/viem-adapter.spec.ts
• npm run check-types
• npx biome check src/provider/viem-adapter.ts test/provider/viem-adapter.spec.ts

Additional local notes:

• npm ci currently fails because package.json and package-lock.json are out of sync on main (eslint@10.8.1 and transitive deps missing from the lockfile). I used npm install --package-lock=false for local verification without changing the lockfile.
• npm run test reports 39 passed test files / 858 passed tests, then exits non-zero due to two unhandled ERR_REQUIRE_ESM errors from @asamuzakjp/css-color requiring @csstools/css-calc under the local Node v21.6.1 environment. The targeted viem adapter test passes.
• npm run lint reports existing repository-wide CRLF formatting differences; the two changed files pass npx biome check directly.