Fix API method guards

[sueun-dev]

Summary

• return immediately from API routes when the request method is not supported
• include Allow headers for the supported methods
• avoid calling Etherscan from /api/getContract after rejecting a non-GET request

Validation

• reproduced the existing /api/getContract behavior: POST returned 200 and still called Etherscan
• verified the patched behavior: unsupported methods return 405 and do not continue handler work
• pnpm run typecheck
• pnpm exec eslint pages/api/getContract.ts pages/api/getDynamicDoc.ts
• pnpm exec prettier --check pages/api/getContract.ts pages/api/getDynamicDoc.ts
• pnpm run build

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