test(internal): add comprehensive tests and improve documentation

[Dark-Brain07]

What changed? Why?

This PR improves test coverage and documentation across the OnchainKit codebase:

1. Added tests for isApiError utility - This internal utility was previously untested. Added comprehensive test suite covering valid API error responses, edge cases (null, undefined, empty objects), and non-error types. Also added JSDoc documentation with usage examples.

2. Added tests for prefixStringParts utility - Added missing test coverage for this string manipulation utility, including tests for basic prefixing, whitespace handling, edge cases, and CSS class name patterns.

3. Enhanced getSwapErrorCode with additional JSON-RPC error codes - Addressed the TODO comment by adding support for -32603 (Internal JSON-RPC error) and -32000 (Server error). Added INTERNAL_ERROR_CODE and SERVER_ERROR_CODE constants with comprehensive JSDoc documentation.

4. Improved JSDoc documentation for core utilities - Enhanced isBase and isEthereum functions with proper @param, @returns, and @example tags to improve developer experience.

5. Added edge case tests for formatPercent utility - Expanded test coverage to include very small numbers, very large values, special values (Infinity, -Infinity, NaN), high precision decimal places, and rounding behavior verification.

Notes to reviewers

• All new tests follow existing testing patterns using Vitest
• The getSwapErrorCode change addresses an existing TODO in the codebase
• Documentation improvements use standard JSDoc format consistent with the rest of the codebase
• Tests are organized into logical describe blocks for better readability

How has it been tested?

• All changes include comprehensive unit tests
• Run pnpm f:ock test to verify all tests pass
• Run pnpm f:ock lint and pnpm f:ock typecheck to verify code quality

[cb-heimdall]

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