refactor(tests): Eliminate test flakiness through deterministic implementation

[balazs-szucs]

Description of Changes

Updated test files to use fixed string identifiers and timestamps instead of random UUIDs and system-dependent times. These changes make the tests more deterministic and easier to debug.

Test determinism and clarity improvements:

• Replaced randomly generated UUIDs with fixed string identifiers in test cases for FileStorageTest.java and TaskManagerTest.java to ensure predictable test data.
• Changed usages of System.currentTimeMillis() and Instant.now() to fixed values in tests for TempFileCleanupServiceTest.java, FileMonitorTest.java, and TextFinderTest.java to avoid flakiness due to timing issues.
• Improved code clarity by adding explanatory comments for time offsets and by using direct string comparisons instead of equals() where appropriate.
• Refactored a timeout simulation in JobExecutorServiceTest.java to use busy-waiting instead of Thread.sleep, reducing test flakiness and improving reliability.

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Checklist

General

• I have read the Contribution Guidelines
• I have read the Stirling-PDF Developer Guide (if applicable)
• I have read the How to add new languages to Stirling-PDF (if applicable)
• I have performed a self-review of my own code
• My changes generate no new warnings

Documentation

• I have updated relevant docs on Stirling-PDF's doc repo (if functionality has heavily changed)
• I have read the section Add New Translation Tags (for new translation tags only)

UI Changes (if applicable)

• Screenshots or videos demonstrating the UI changes are attached (e.g., as comments or direct attachments in the PR)

Testing (if applicable)

• I have tested my changes locally. Refer to the Testing Guide for more details.

[Copilot]

Pull Request Overview

Refactors tests to reduce flakiness by eliminating randomness and system-time dependencies, standardizing string identifiers, and clarifying time-based logic. Also disables a particularly flaky test and adjusts a timeout simulation approach.

• Replace random UUIDs with deterministic string IDs in tests
• Swap time-dependent calls with fixed or clearer time values and comments
• Disable a flaky recursive cleanup test; change a timeout simulation from sleep to a busy-wait loop

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
app/core/src/test/java/stirling/software/SPDF/pdf/TextFinderTest.java	Use fixed start/end timestamps to avoid timing flakiness.
app/common/src/test/java/stirling/software/common/util/FileMonitorTest.java	Set deterministic lastModifiedTime values for test files/dirs.
app/common/src/test/java/stirling/software/common/service/TempFileCleanupServiceTest.java	Clarify comments, NPE-safe string comparisons, fixed timestamps in one test, and disable a flaky test.
app/common/src/test/java/stirling/software/common/service/TaskManagerTest.java	Replace UUIDs with fixed job IDs.
app/common/src/test/java/stirling/software/common/service/ResourceMonitorTest.java	Clarify relative time via comment.
app/common/src/test/java/stirling/software/common/service/JobExecutorServiceTest.java	Replace Thread.sleep with a busy-wait to simulate long-running work.
app/common/src/test/java/stirling/software/common/service/FileStorageTest.java	Replace UUIDs with fixed file IDs.

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