This PR introduces a FakeTwilioClient testing utility that solves the long-standing challenge of testing applications that use Twilio services. The fake client provides a drop-in replacement for the real Twilio client that validates parameters, returns configurable responses, and tracks API calls without making actual HTTP requests.

Problem Solved

Testing Twilio integrations has been difficult because existing approaches have significant drawbacks:

• Mocking the entire client fails to catch problems with invalid parameters or function names
• Using localhost/mock servers requires reverse-engineering the Twilio protocol
• Mocking HTTP requests is brittle and breaks when the client changes
• Using test credentials makes tests non-hermetic, slow, and dependent on network connectivity

Solution

The FakeTwilioClient addresses all these issues by:

✅ Validating parameters like the real client - catches TypeError for missing required params, ValueError for invalid combinations
✅ Returning configurable fake responses - test success/failure scenarios without network calls
✅ Tracking API calls - verify your code makes the expected calls with correct parameters
✅ Working as drop-in replacement - inherits from real Client class for full compatibility

Usage Examples

Basic usage:

from twilio.testing import FakeTwilioClient

fake_client = FakeTwilioClient()
message = fake_client.messages.create(
    to="+15551234567",
    from_="+15559876543", 
    body="Test message"
)

assert message.status == "sent"
assert len(fake_client.get_calls()) == 1

Parameter validation catches bugs:

# Missing required 'to' parameter - raises TypeError
fake_client.messages.create(from_="+15559876543", body="Missing to")

# Invalid parameter combination - raises ValueError  
fake_client.messages.create(to="+15551234567", body="No sender specified")

Custom responses for different scenarios:

fake_client.configure_response("messages.create", {
    "sid": "SM123456789",
    "status": "failed",
    "error_code": 21211, 
    "error_message": "Invalid phone number"
})

Call verification:

fake_client.assert_called_with("messages.create",
    To="+15551234567",
    Body="Expected message content"
)

Files Added

• twilio/testing/ - New testing utilities module
• twilio/testing/fake_client.py - Main FakeTwilioClient implementation
• tests/unit/testing/test_fake_client.py - Comprehensive test suite (16 tests)
• docs/testing.md - Complete documentation with examples
• examples/testing_example.py - Real-world usage example
• TESTING.md - Quick start guide for easy discovery

Benefits

This implementation provides a much better testing experience than existing approaches:

• Better test coverage - Parameter validation catches more bugs than simple mocking
• Faster tests - No network dependencies mean tests run quickly and reliably
• Easier maintenance - No need to understand HTTP protocol details or maintain mock servers
• More realistic - Tests work with the actual client API surface

Fixes #7.

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