Add comprehensive integration tests for contacts management endpoints

[Copilot]

This PR implements a complete integration test suite for the contacts management API endpoints at /api/v1/contacts/, providing comprehensive coverage of all CRUD operations, search functionality, relationships, data validation, and security measures.

What's Added

49 Integration Tests Across 9 Test Classes

Core Functionality:

• CRUD Operations (11 tests) - Complete Create, Read, Update, Delete testing with proper validation, error handling, and edge cases
• Search & Pagination (6 tests) - Text search across contact fields with pagination, special characters, Unicode support, and performance testing
• Listing & Pagination (4 tests) - Contact listing with empty states, data presence, and pagination parameters

Security & Data Integrity:

• Authentication & Authorization (7 tests) - Unauthenticated access prevention, invalid token handling, and proper authorization validation
• Tenant Isolation (1 test) - Multi-tenant data security ensuring proper tenant context enforcement
• Data Validation (6 tests) - SQL injection protection, XSS prevention, Unicode support, and field validation
• Advanced Security (3 tests) - Field length limits, email format variations, and concurrent access scenarios

Relationships & Integration:

• Contact-Event Relationships (4 tests) - Testing event associations, shared events between contacts, and relationship queries
• Cascade Operations (2 tests) - Verifying proper cascade delete behavior for contact interactions and related data

Performance & Data Integrity:

• Bulk Operations (2 tests) - Multiple contact creation and search performance with larger datasets
• Data Integrity (3 tests) - Consistency validation, duplicate handling, and null/empty field behavior

Bug Fixes Applied

During test development, discovered and fixed critical issues in the existing codebase:

1. Schema-Model Mismatch: The ContactBase schema defined title, company, and notes fields that didn't exist in the Contact model, causing runtime errors during contact creation. Aligned the schema with the actual model structure.

2. Search Function Bug: The search_contacts function was attempting to query a non-existent company field, causing database errors. Updated to search only on existing full_name and email fields.

Endpoints Tested

All contacts management endpoints are now comprehensively tested:

• GET /api/v1/contacts/ - List contacts with pagination
• GET /api/v1/contacts/search/ - Search contacts by name/email
• GET /api/v1/contacts/{contact_id} - Get contact details
• POST /api/v1/contacts/ - Create new contacts
• PUT /api/v1/contacts/{contact_id} - Update contacts
• DELETE /api/v1/contacts/{contact_id} - Delete contacts
• GET /api/v1/contacts/{contact_id}/events - Get contact events
• GET /api/v1/contacts/{contact_id}/shared-events/{other_contact_id} - Get shared events

Technical Implementation

• Uses pytest framework with proper test isolation
• Custom fixtures for database setup with tenant and user contexts
• Comprehensive error scenario testing
• Performance testing with bulk operations
• Security testing including SQL injection and XSS protection
• Unicode and internationalization support validation

The test suite ensures robust validation of the contacts management system while maintaining minimal changes to the existing codebase, focusing on testing the system as it actually works rather than making unnecessary modifications.

Fixes #42.

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