test: expand backend unit coverage across controllers and services

[thughari]

Motivation

• Increase unit-test coverage across the backend to exercise parsing, validation, security, and storage logic.
• Focus on controller edge cases, service validation branches, security token handling, and small utility/mock components that were previously untested.

Description

• Added comprehensive tests for WebhookController to cover invalid payloads, Google forwarding verification parsing, system-email ignores, successful job extraction, and gemini-skip behavior (backend/src/test/java/com/thughari/jobtrackerpro/controller/WebhookControllerTest.java).
• Added security tests for JWT generation/validation and the auth filter (JwtUtilsTest, JwtAuthenticationFilterTest) using ReflectionTestUtils and mocked JwtUtils.
• Added GlobalExceptionHandlerTest to assert HTTP mapping and ErrorResponse payloads for bad requests, not found, upload-size, and general exceptions.
• Added CareerResourceServiceTest to exercise pagination sanitization, duplicate URL rejection, resource-file creation path, unauthorized update rejection, and file deletion handling.
• Added tests for smaller components: JobSchedulerTest, MockGeminiServiceTest, and LocalStorageServiceTest to cover scheduled cleanup invocation, mock extraction defaults, and local upload/validation rules.

Testing

• Created and added the new unit tests under backend/src/test/java/... and validated test sources compile locally in the editor.
• Attempted to run the full backend suite with mvn test and ./mvnw test, but the build failed to resolve the parent Spring Boot starter POM from Maven Central (HTTP 403), so the test run did not complete.
• No additional automated test runs succeeded in this environment due to external dependency resolution being blocked.

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Codex Task