Move `app/nodes/plan_actions/detect_sources_test.py` into `tests/nodes/plan_actions/`

[VaibhavUpreti]

Why This Matters

node coverage is easier to navigate when all node tests live under tests/nodes/.

Read First

• app/nodes/plan_actions/detect_sources_test.py
• tests/nodes/plan_actions/

Scope

1. Move the file into tests/nodes/plan_actions/.
2. Keep the GitLab merge_request_iid assertions intact.
3. Delete the inline copy.

Tests / Coverage

• no new assertions are required
• first prove the moved file still collects correctly

Acceptance Criteria

• source-detection tests live under tests/nodes/plan_actions/
• the inline file is removed
• behavior is unchanged

[Davidson3556]

can I be assigned this? @VaibhavUpreti