Add a Pester test suite for the collectors

84 tests in collectors/tests, run with Invoke-CollectorTests.ps1 (~15 s,
PowerShell 7.2+ and Pester 5.5+):

- Module.Tests: the shared module (property lookup, time format,
  config/lookback, sent-run state and pruning, dry runs, exit codes,
  401 detection)
- Veeam/Mars/Wsb/Vb365.Tests: each collector run in its own process
  against its mock and a stand-in ingest API, asserting the behaviours
  confirmed on real servers: VBR 13 live schedule, copy jobs, MARS UTC
  times and ErrorCode 0, WSB culture-formatted times and nested VMs,
  VB365 organization mapping, log prefixes and retries, state and
  resending, rejected keys
- Installers.Tests: config, files and scheduled task from each
  installer, with stand-ins for icacls / Task Scheduler / the key prompt

Collectors run under the test runner's PowerShell, or another shell via
-CollectorShell (e.g. powershell.exe, to test under Windows PowerShell
5.1 like the MARS, WSB and VB365 servers). The mocks take their "now"
from MOCK_NOW so expectations can't drift with the clock; the WSB mock
gains -RunningJobFinished for the running-to-finished case.

Each of 12 past bugs, put back one at a time, fails the test for its
behaviour (and the runner exits 1).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

README.md

@@ -138,6 +138,12 @@ BACKUP_DASH_SERVE_STATIC=1 uvicorn --factory app.main:create_app --reload
 # → http://localhost:8000
 ```
 
+Collector tests (PowerShell 7 + Pester; see [collectors/tests](collectors/tests/README.md)):
+
+```powershell
+./collectors/tests/Invoke-CollectorTests.ps1
+```
+
 ## Deployment (Linux, nginx)
 
 1. Copy the repo to `/opt/backup-dashboard`, create a venv at `/opt/backup-dashboard/venv`, and run `pip install -r backend/requirements.txt`.