feat(schemas): enhance is_successful logic to handle zero-padded success codes

[RafaelJohn9]

Description

This PR addresses inconsistent handling of M-Pesa success codes across different API schemas. Previously, is_successful() methods only checked for exact string equality with "0", but M-Pesa APIs sometimes return success codes as "00000000" or other zero-padded variations. This caused false negatives in transaction status checks.

The changes implement a robust success detection logic that treats any string consisting entirely of zeros (e.g., '0', '00000000') as successful, while properly rejecting mixed codes like '00001' or empty strings.

Fixes #88

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Tests (addition or improvement of tests)

How Has This Been Tested?

• Added comprehensive unit tests for all affected schemas (B2BExpressCheckout, B2C, Reversal, TransactionStatus)
• Test cases cover:
  • Single zero ("0")
  • Multiple zeros ("00000000")
  • Non-zero codes ("1", "-1")
  • Mixed codes ("00001")
  • Empty strings
  • Both string and integer code inputs
• Verified that existing functionality remains unchanged for standard "0" success codes
• All tests pass locally with both Python 3.9 and 3.12

Checklist

• My code follows the project's coding style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Additional Context

This fix ensures consistent behavior across all M-Pesa API integrations in the SDK. The implementation uses code.strip("0") == "" and code != "" which efficiently handles all zero-only strings while rejecting edge cases like empty strings or whitespace-only inputs.

The change is backward compatible since "0" continues to work exactly as before, and it aligns with M-Pesa's documented behavior where success is indicated by any variation of zero codes.

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