fix: use lazy imports to speed up module import time

[BinoyOza-okta]

feat: Implement lazy loading to reduce SDK import time from ~2s to milliseconds

This change addresses a critical performance issue where importing the okta-sdk-python
module took nearly 2 seconds instead of milliseconds due to eager importing of all
models and API classes.

Problem:

• Initial import of okta module was loading 1000+ model classes and 100+ API classes eagerly
• Each model had complex Pydantic validation and inheritance chains
• Import time was ~2 seconds, blocking application startup
• Related to issue Module import takes nearly two seconds #476

Solution:
Implemented intelligent lazy loading across three levels:

1. Package Level (okta/__init__.py):

   • Lazy load API classes and models using __getattr__ hook
   • Only core SDK components (ApiClient, Configuration, etc.) are eagerly loaded
   • Models and API classes loaded on-demand when first accessed

2. API Level (okta/api/__init__.py):

   • Lazy load individual API classes using __getattr__ pattern
   • Thread-safe imports with locking mechanism
   • Maintains backward compatibility

3. Models Level (okta/models/__init__.py):

   • Dynamic model grouping based on discriminator relationships
   • Preloads dependent models together to maintain class identity consistency
   • Critical for Pydantic validation with polymorphic models
   • Groups auto-generated from OpenAPI discriminator mappings
   • Manual field dependencies for complex model relationships

Key Technical Changes:

• Template Updates:

  • __init__model.mustache: Added lazy loading with discriminator-based grouping
  • __init__package.mustache: Implemented __getattr__ for lazy loading
  • api.mustache: Fixed Content-Type header handling for non-consumes endpoints
  • model_generic.mustache: Import discriminator children from models package for class identity

• Model Grouping Strategy:

  • Auto-groups models with parent-child discriminator relationships
  • Manual groups for field-level dependencies (e.g., DeviceAssurance → OSVersion)
  • Prevents Pydantic validation errors from duplicate class instances
  • Thread-safe lazy loading with locking

• Generated Code Changes:

  • All API classes updated with lazy import support
  • All model classes updated to use centralized imports for discriminator handling
  • Models now import from okta.models instead of direct file paths

Testing:

• Updated GitHub Actions workflow to run pytest
• Removed outdated commented test code in test_applications_it.py
• All integration tests passing with new lazy loading implementation
• Thread-safe implementation prevents race conditions in concurrent imports

Performance Impact:

• Import time: ~2000ms → <100ms (20x improvement)
• No runtime performance impact after initial load
• Maintains full backward compatibility
• Models loaded on-demand only when used

Backward Compatibility:

• All existing import patterns continue to work
• from okta.models import ModelName still functional
• from okta import ApiClass still functional
• No breaking changes to public API

Fixes #476