docs(evm): clarify channel id and transfer method defaults

[EfeDurmaz16]

Summary

• fix the batch-settlement voucher channelId docstring to reference the EIP-712 computeChannelId derivation instead of keccak256(abi.encode(ChannelConfig))
• clarify that EVM exact extra.assetTransferMethod can be omitted from PaymentRequired for default method selection
• add notes to the v2 HTTP and core examples explaining why their extra objects include token domain metadata but no transfer-method selector

Why

This addresses #2323. The previous wording could send external implementers toward an incompatible channel id derivation, and the EVM exact examples made assetTransferMethod look required even though canonical PaymentRequired examples omit it.

Verification

• git diff --check
• pnpm exec prettier -c .prettierrc --check src/batch-settlement/client/voucher.ts ../../../../specs/schemes/exact/scheme_exact_evm.md ../../../../specs/transports-v2/http.md ../../../../specs/x402-specification-v2.md from typescript/packages/mechanisms/evm
• pnpm --filter @x402/evm lint:check

Notes

• pnpm --filter @x402/evm test -- --runInBand currently fails locally before exercising this docs change because the package-level Vitest run cannot resolve workspace imports such as @x402/core/http, @x402/core/client, and @x402/core/utils in this checkout. The collected unit tests that did not hit those aliases passed.

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[EfeDurmaz16]

Closing this as a duplicate of #2325. #2325 already addresses #2323 with the channelId docstring fix and EVM exact assetTransferMethod defaulting clarification; keeping one PR open is cleaner for review.