Add unit tests for chat and private chat services

[vizahat36]

Related to #109

This pull request adds comprehensive unit tests for the Chat Management service layer, improving coverage of chat-related business logic and critical edge cases.

What was added:

• ChatServiceTest: covers saving group and private messages, sender validation, and encryption/decryption logic
• PrivateChatServiceTest: covers retrieving existing chats (both directions), creating new chats, and user-not-found edge cases

Coverage summary:

• Chat management: create, retrieve, and validate chat messages
• Edge cases: missing users and existing chat detection
• Tests use JUnit 5 and Mockito
• All tests are isolated, fast, and do not depend on DB or network

Notes:

• User management tests were added in a previous PR
• Group management tests will be handled separately

Test verification:

• ./mvnw test passes successfully (42 tests, 0 failures)

[vizahat36]

@DenizAltunkapan
This PR is intentionally scoped to Chat Management unit tests in alignment with Issue #109.
User management was addressed in a prior PR, and group management tests will be submitted separately.

Please let me know if any additional coverage or refinements are required.

[Copilot]

Pull request overview

This pull request adds comprehensive unit tests for the Chat Management service layer, specifically for ChatService and PrivateChatService. The tests improve coverage of chat-related business logic including message saving, encryption/decryption, and private chat retrieval/creation.

Changes:

• Added ChatServiceTest with 10 test methods covering message saving, encryption/decryption, and various error scenarios
• Added PrivateChatServiceTest with 5 test methods covering private chat retrieval, creation, and user validation
• Tests use JUnit 5 and Mockito for isolated, fast unit testing without database dependencies

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
backend/src/test/java/vaultWeb/services/ChatServiceTest.java	Comprehensive tests for ChatService covering group/private message saving, sender validation, encryption/decryption logic, and error handling
backend/src/test/java/vaultWeb/services/PrivateChatServiceTest.java	Tests for PrivateChatService covering bidirectional chat lookup, creation, and user not found scenarios

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[DenizAltunkapan]

good job, as always thank you! @vizahat36