fix: infer EIP-712 primaryType in viem adapter

[Mabolla]

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Summary

Fixes EIP-712 signing through the SDK's standard viem wallet adapter when a dependency struct appears before the actual root struct in the types object.

This is the same primary-type inference issue addressed for the separate Seaport bridge path in #1998, but src/provider/viem-adapter.ts has its own signTypedData() implementation and retained the same first-key heuristic.

Root cause

createViemSigner().signTypedData() selected the first non-EIP712Domain key as primaryType.

EIP-712 does not require the root struct to be declared first. For example, when Person is declared before Mail and Mail references Person, the correct primary type is Mail, but the viem adapter selected Person.

Changes

• Infer the root EIP-712 struct by finding the named type that is not referenced by another struct.
• Handle struct references through array types such as Person[].
• Preserve the existing first-type fallback for cyclic or ambiguous type graphs.
• Add a regression test specifically against the standard viem wallet adapter path.

Validation

• Regression test fails before the fix because Person is selected instead of Mail.
• Targeted regression test: 1/1 passed after the fix.
• Full unit suite: 946/946 passed across 42 test files.
• Targeted Biome check passes for the changed files.

Related

#1998 fixes the separate Seaport bridge implementation. This PR covers the standard viem adapter implementation in src/provider/viem-adapter.ts.