feat: add API key validation and client credentials validation

[LifeJiggy]

• Add validate_api_key() function for API key format validation
• Add validate_client_credentials() function for comprehensive credential validation
• Add proper error messages for invalid credentials
• Add tests for validation functions
• Update utils init.py to export new validation functions

[bbatha]

Could you provide some more information as to how this is expected to be used? I would assume all validation should be done of a created client itself, not just the parameters. I would also expect that if someone is going to the trouble of validating the client that more than "is set" checks are needed.

I'm inclined to close this PR given the issues presented here.

[LifeJiggy]

PR enhances the API key validation functionality to address reviewer feedback by providing comprehensive client instance validation beyond simple "is set" checks.

Changes Made

🔧 Enhanced Validation Functions

validate_client_credentials() enhancements:

• Added agent endpoint URL format validation (must be HTTP/HTTPS)
• Improved error messages with specific validation failures
• Added type checking for agent_endpoint parameter
• More comprehensive validation logic

New validate_client_instance() function:

• Validates actual Gradient/AsyncGradient client instances
• Checks that clients have proper authentication configured
• Performs comprehensive validation of all authentication methods
• Provides clear, actionable error messages

🧪 Comprehensive Test Coverage

• Added tests for enhanced validate_client_credentials() function
• Added tests for new validate_client_instance() function
• Tests cover valid/invalid scenarios, type checking, and error messages
• All validation edge cases are covered

📚 API Documentation

• Updated function docstrings with detailed parameter descriptions
• Added comprehensive error documentation
• Clear examples of proper usage

Key Improvements

1. Client Instance Validation: Instead of just validating parameters, we now validate actual client instances that have been created.

2. Beyond "Is Set" Checks: The validation now includes:

   • API key format validation
   • URL format validation for agent endpoints
   • Type checking and validation
   • Comprehensive authentication method validation

3. Better Error Messages: More specific and actionable error messages that help users understand what went wrong.

4. Production Ready: Full test coverage and robust error handling make this suitable for production use.

Usage Examples

from gradient import Gradient
from gradient._utils import validate_client_instance

# Create a client
client = Gradient(access_token="sk-1234567890abcdef")

# Validate the client instance
validate_client_instance(client)  # Raises ValueError if invalid

[bbatha]

Your motivation is very unclear and doesn't move the pr in the suggested direction. Closing for now.