feat(config): add startup validation to fail fast on invalid secrets

[abdrassulov]

📌 Description

This PR implements fail-fast startup configuration validation for critical configuration secrets, preventing runtime failures and timing/parsing bugs later.

✅ Solution

• Added a new method to the struct in .
• Validates that and are present.
• Validates that is valid base64 and decodes to exactly 32 bytes (required for AES-256-GCM OAuth token encryption at rest).
• Validates that Soroban secrets (, , , contract IDs) are present and valid, including checking that is a valid Stellar secret key (starts with "S" and is 56 characters long).
• Integrated the validation checks into and entrypoints.
• Created a comprehensive unit test suite in covering all validation cases.

🧪 Test Results

All unit tests pass successfully!