Add unit tests for all controllers

[osvaldoandrade]

Test all 15 HTTP controllers individually.

Current coverage:

• Only batch_controllers_test.go (568 LOC)

Goal:

• Unit test per controller

Implementation:

• Create test files:
  • create_task_controller_test.go
  • claim_task_controller_test.go
  • submit_result_controller_test.go
  • heartbeat_controller_test.go
  • etc. (15 total)
• Use table-driven tests
• Mock application layer (simple fakes)

Acceptance criteria:

• Each controller has dedicated test file
• Coverage ≥90% on controllers
• Tests isolated (no external dependencies)

Part of: #654
Requires: #649 (controllers regrouped), #654 (testutil/wait)