add optional session parameter to CodeInterpreter for custom credential management

[gonzalo123]

Add boto3.Session injection support to CodeInterpreter constructor to enable custom credential configurations and role assumption while maintaining full backward compatibility.

Features:

• Optional session parameter in CodeInterpreter constructor
• Support for pre-configured boto3 sessions with custom credentials
• Enable AWS role assumption and cross-account access
• Session reuse across multiple service clients
• Full backward compatibility with existing implementations

Use Cases:

• Dynamic role switching for multi-tenant applications
• Custom credential configurations beyond environment variables
• Cross-account resource access with assumed roles
• Containerized environments with temporary credentials
• Testing scenarios with mock sessions
• Fine-grained credential management and security

Configuration:

• Compatible with all existing CodeInterpreter functionality

Implementation:

• Updated CodeInterpreter.init() to accept optional session parameter
• Modified code_session() context manager to support session injection
• Session validation and client creation logic preserved
• Comprehensive test coverage for both default and custom session scenarios
• Updated documentation with usage examples and best practices

Benefits:

• Enhanced credential flexibility for enterprise applications
• Improved security through controlled session management
• Better testing capabilities with session mocking
• Simplified multi-account and role-based access patterns
• Maintains existing API contract and behavior

Breaking Changes: None

• Default behavior remains identical to previous implementation
• All existing code continues to work without modification
• New functionality only active when session parameter is provided

CLOSES #40