Implement comprehensive ESP32 unit test infrastructure

[Copilot]

This PR implements the complete unit test infrastructure described in docs/TESTING.md that was previously missing from the repository.

Problem

The repository included comprehensive testing documentation in docs/TESTING.md describing a complete unit test structure, but the actual test files and infrastructure were completely missing. The existing CI only performed basic build validation without any actual unit testing.

Solution

Implemented the full testing infrastructure as documented:

Test Structure Created

• components/main_test/ - Complete test component following ESP-IDF conventions
• components/main_test/test_main.c - Main test runner with Unity framework
• components/main_test/test_utils.c - Utility function tests (WiFi mode conversion, JSON validation, phone number validation)
• components/main_test/test_http_handlers.c - HTTP endpoint tests with mocking
• components/main_test/test_nvs_utils.c - NVS storage operation tests
• components/main_test/test_utils.h - Test declarations and mock structures

Test Coverage

• HTTP Handlers: All endpoints tested (/dial, /redial, /status, /configure_wifi, /set_auto_redial)
• JSON Processing: Parsing, validation, and error handling for configuration requests
• NVS Operations: WiFi credentials and auto-redial settings storage/retrieval with error handling
• Utility Functions: WiFi mode string conversion, status response generation
• Error Scenarios: Bluetooth disconnected, invalid JSON, missing parameters, boundary conditions

Testing Infrastructure

• Proper mocking strategy for ESP32 hardware dependencies (HTTP server, NVS, Bluetooth)
• Unity test framework integration
• Separate test project (test/) with ESP-IDF structure for building and running tests
• Test-specific configuration (test/sdkconfig.defaults)

CI/CD Integration

• Enhanced .github/workflows/test-firmware.yml to build and validate unit tests
• Automatic test execution on PRs affecting firmware code
• Code quality checks now include test file validation

Usage

# Navigate to test directory
cd test

# Build the test project
idf.py build

# Flash and monitor (with ESP32 hardware)
idf.py flash monitor

The implementation follows the exact structure described in docs/TESTING.md and provides comprehensive coverage of the ESP32 application's business logic without requiring physical hardware.

Fixes #20.

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