feat: implement credential deletion on 401 responses

[devin-ai-integration]

Add credential deletion functionality to handle 401 responses

This PR implements the credential deletion functionality in the Neon CLI tool to handle 401 responses.

Changes

• Added deleteCredentials(configDir: string): void function in src/commands/auth.ts
• Added new error code 'CREDENTIALS_DELETE_FAILED' in src/errors.ts
• Added comprehensive tests for the new functionality

Testing

• Added tests for successful credentials file deletion
• Added tests for handling non-existent files
• Added tests for handling permission errors

Link to Devin run: https://app.devin.ai/sessions/15efddb4c6734636ab10bbfc62a1e47e
Requested by: Konstantin Merenkov

[devin-ai-integration]

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