chore(frontend): remove the unused WorkflowSnapshotService

[aglinxinyuan]

What changes were proposed in this PR?

Deletes WorkflowSnapshotService, which calls REST endpoints that no longer exist, and the model type used only by it. Pure deletion, no behaviour change: −100 lines.

The service targets ${AppSettings.getApiEndpoint()}/snapshot — PUT /snapshot/upload and GET /snapshot/{sid}. There is no @Path("/snapshot") resource anywhere in the Scala sources, so both requests would 404 if anything invoked them.

Nothing does. The service's only references are in execute-workflow.service.spec.ts, which imports it and calls TestBed.inject but never asserts on it — an inert injection, removed here along with the import and its field. WorkflowSnapshotEntry is used by the service and nowhere else.

Reviewer note: html2canvas stays — report-generation.service.ts uses it independently of this service.

Any related issues, documentation, discussions?

Closes #7618

How was this PR tested?

Existing tests only — this PR adds none; it removes a service nothing exercised.

Locally, from frontend/:

• npx ng test --watch=false --include='**/execute-workflow.service.spec.ts' — 35 tests, all pass after dropping the inert injection.
• yarn --cwd frontend format:ci — clean.

Verification, re-runnable by a reviewer:

git grep -n WorkflowSnapshotService             # only the deleted service after this change
git grep -n '@Path("/snapshot'  -- '*.scala'    # no backend route exists

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.90%. Comparing base (c35bdb1) to head (e5ff1b6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #7621      +/-   ##
============================================
+ Coverage     88.89%   88.90%   +0.01%     
  Complexity     4348     4348              
============================================
  Files          1179     1178       -1     
  Lines         46919    46906      -13     
  Branches       5230     5227       -3     
============================================
- Hits          41707    41702       -5     
+ Misses         3463     3455       -8     
  Partials       1749     1749              

Flag	Coverage Δ		*Carryforward flag
access-control-service	70.00% <ø> (ø)		Carriedforward from c35bdb1
agent-service	98.62% <ø> (ø)		Carriedforward from c35bdb1
amber	84.70% <ø> (ø)		Carriedforward from c35bdb1
computing-unit-managing-service	60.38% <ø> (ø)		Carriedforward from c35bdb1
config-service	77.31% <ø> (ø)		Carriedforward from c35bdb1
file-service	68.90% <ø> (ø)		Carriedforward from c35bdb1
frontend	90.61% <ø> (+0.03%)	⬆️
notebook-migration-service	78.89% <ø> (ø)		Carriedforward from c35bdb1
pyamber	97.52% <ø> (ø)		Carriedforward from c35bdb1
workflow-compiling-service	57.89% <ø> (ø)		Carriedforward from c35bdb1

*This pull request uses carry forward flags. Click here to find out more.

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