test : added unit tests for lib/github-token-encryption module - #8

Summary of What Has Been Done

Created lib/github-token-encryption.test.ts with comprehensive unit tests covering:

encryptGitHubToken / decryptGitHubToken:

• Produces valid GCM-format output (4 dot-separated base64 parts)
• Round-trips correctly (encrypt then decrypt returns original)
• Throws on empty string and non-string inputs
• Throws on unrecognized encrypted token format
• Tampered ciphertext causes decryption to throw
• Different encryptions produce different ciphertexts (random IV)

parseAndEncryptTokens:

• Encrypts multiple comma-separated tokens with sequential rotation indices
• Throws on empty/whitespace-only string
• Filters out empty tokens between commas
• Warns on non-GitHub token prefixes
• Does not warn for ghp_ and ghu_ prefixes

getNextToken:

• Returns correct token and cycles through the array
• Throws when no tokens are provided

isEncryptedToken:

• Returns true for valid GCM format
• Returns false for plain text, null, undefined, and empty string
• Validates base64 validity of each GCM part

redactToken:

• Shows first 4 + last 4 characters with "..." separator
• Returns "***" for tokens shorter than 10 characters
• Handles null and undefined safely

Mocked server-only and set GITHUB_TOKEN_ENCRYPTION_KEY in the test environment.

Changes Made

• New file: lib/github-token-encryption.test.ts

Impact it Made

Adds direct unit test coverage for the GitHub token encryption module, ensuring security-critical logic is robust against edge cases.

Closes #8423.

Note: please assign this PR to the tmdeveloper007 account.