feat: improve transaction safety in CompleteJob function (coder#17970)

This PR refactors the CompleteJob function to use database transactions
more consistently for better atomicity guarantees. The large function
was broken down into three specialized handlers:

- completeTemplateImportJob
- completeWorkspaceBuildJob
- completeTemplateDryRunJob

Each handler now uses the Database.InTx wrapper to ensure all database
operations for a job completion are performed within a single
transaction, preventing partial updates in case of failures.

Added comprehensive tests for transaction behavior for each job type.

Fixes coder#17694

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Co-authored-by: Claude <noreply@anthropic.com>

Author: EdwardAngert