chore: relocate file-service

[Yicong-Huang]

According to the reorganization plan #3846, this PR moves core/file-service out to the root folder.
resolves #3858.

[Copilot]

Pull Request Overview

This PR implements a reorganization plan to move service modules out of the core directory structure to the root level. The changes remove utility path methods from PathUtils and update service configurations to use direct path resolution instead.

Key changes:

• Remove path utility methods for services that were moved out of core
• Update service configuration paths to use direct Path.of() resolution
• Update test file paths to reflect new directory structure

Reviewed Changes

Copilot reviewed 5 out of 30 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
workflow-compiling-service/src/test/scala/org/apache/texera/service/resource/WorkflowCompilationResourceSpec.scala	Updates test resource file path to reflect moved service location
workflow-compiling-service/src/main/scala/org/apache/texera/service/WorkflowCompilingService.scala	Replaces PathUtils usage with direct path resolution for configuration file
file-service/src/main/scala/org/apache/texera/service/FileService.scala	Replaces PathUtils usage with direct path resolution and cleans up imports
core/config/src/main/scala/org/apache/amber/util/PathUtils.scala	Removes path utility methods for services moved out of core
computing-unit-managing-service/src/main/scala/org/apache/texera/service/ComputingUnitManagingService.scala	Replaces PathUtils usage with direct path resolution for configuration file

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