Add email validation to remove_access and improve test coverage

[DecentMakeover]

Summary

This PR fixes #347 by enforcing email validation across all Shared Access APIs and adding comprehensive test coverage.

Changes

• Add email validation to ShareChatService.remove_access()
• Add 18 unit tests for ShareChatService, covering:
  • Valid and invalid email formats in share_chat and remove_access
  • Edge cases: empty emails, whitespace, malformed formats
  • Error handling: non-existent conversations and unauthorized access
• Add 30+ unit tests for email_helper.is_valid_email()
• Ensure consistent validation across all sharing endpoints

Test Coverage

The tests include:

• Valid email formats (dots, plus signs, numbers, subdomains, etc.)
• Invalid formats (missing @, domain, TLD, spaces, invalid characters, etc.)
• Edge cases (very long emails, empty strings, whitespace-only, etc.)

Summary by CodeRabbit

• Bug Fixes

  • Stricter email validation for sharing and access removal; invalid addresses now return clear 400 errors before any changes.
  • Ensures atomic operations: if any email is invalid, no updates are made.

• Documentation

  • Expanded endpoint docs detailing email rules, error cases, and atomic behavior.

• Tests

  • Comprehensive tests for valid/invalid email formats and share/remove access flows to ensure correct behavior and prevent partial updates.