test(frontend): cover the joint graph wrapper's layout and co-editor paths

[aglinxinyuan]

What changes were proposed in this PR?

joint-graph-wrapper.ts sat at 76.4% lines and 50.6% branches. The branch number is the one
that mattered — half its conditions had only ever been taken one way.

Adds 20 tests to the existing spec, appended as new blocks rather than edits so a rebase stays
cheap (#6489 also touches this file). Covered: absolute positioning, the link-cell change stream,
auto layout, the co-editor presence rings including re-padding when one is deleted, the editing
banner, and the guard clauses for missing and wrong-typed cells.

	Before	After
Lines	220/288 (76.4%)	282/288 (97.91%)
Branches	43/85 (50.6%)	82/85 (96.47%)
Functions	98/111 (88.3%)	107/111 (96.39%)

Verification

46 mutations were applied and reverted, 45 red on the first pass. One survived — the
currentStrokeIds.includes(highlightIdToDelete) guard — and the test was strengthened to assert
ring order after a ghost delete, which kills it.

Review then found two more problems, both fixed:

• A test whose name outran its assertions. "removeCurrentEditing hides the banner and stops the animation" could not fail on the second half: removeCurrentEditing blanks the banner text
  first, so the interval body's ownership check is permanently false and advancing the timers is a
  no-op whether or not the interval was cleared. clearInterval is already pinned by a
  pre-existing test, so the unfalsifiable half was removed rather than propped up.
• A comment that misdescribed its own branch. It claimed a link exercised the || 0 fallback in
  getCellLayer; a link's z is explicitly 0, so it only proves a present zero survives.

Deliberately not included

getCellLayer's || 0 fallback (line 752) is unreachable, and no test pins it. Mutating it to
?? -1 survives, and the reason is structural rather than a missing test: joint's Graph.addCell
sets z = maxZIndex() + 1 on any cell that arrives without one, and getCellLayer throws for a
cell that is not in the graph — so attributes.z is never undefined where it is read. Deleting
the || 0 outright does not compile (z is number | undefined), so it is load-bearing for typing
only. That is recorded in the spec so the next reader does not re-derive it.

Also left alone: the link-breakpoint members. jointLinkBreakpointShowStream and
jointLinkBreakpointHideStream are private Subjects never .next()-ed anywhere in frontend/src,
and linksWithBreakpoints is declared and read but never written.

No production file is touched.

Any related issues, documentation, discussions?

Closes #7508

How was this PR tested?

npx ng test --watch=false --include="**/joint-graph-wrapper.spec.ts"

 Test Files  1 passed (1)
      Tests  67 passed (67)

20 new on top of the existing 47. yarn format:ci passes.

Was this PR authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Opus 5)

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[github-actions]

Automated Reviewer Suggestions

Based on the git blame history of the changed files, we recommend the following reviewers:

• Contributors with relevant context: @mengw15
  You can notify them by mentioning @mengw15 in a comment.

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Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.07%. Comparing base (42d08a3) to head (3dd6d7b).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #7509      +/-   ##
============================================
- Coverage     86.38%   86.07%   -0.32%     
+ Complexity     4211     4152      -59     
============================================
  Files          1169     1169              
  Lines         46750    46733      -17     
  Branches       5203     5202       -1     
============================================
- Hits          40387    40224     -163     
- Misses         4641     4821     +180     
+ Partials       1722     1688      -34     

Flag	Coverage Δ		*Carryforward flag
access-control-service	70.00% <ø> (ø)		Carriedforward from 307dfc4
agent-service	85.50% <ø> (-1.38%)	⬇️	Carriedforward from 307dfc4
amber	80.88% <ø> (-0.99%)	⬇️	Carriedforward from 307dfc4
computing-unit-managing-service	50.72% <ø> (ø)		Carriedforward from 307dfc4
config-service	65.97% <ø> (ø)		Carriedforward from 307dfc4
file-service	69.05% <ø> (ø)		Carriedforward from 307dfc4
frontend	89.53% <ø> (+0.30%)	⬆️
notebook-migration-service	78.89% <ø> (ø)		Carriedforward from 307dfc4
pyamber	97.55% <ø> (-0.01%)	⬇️	Carriedforward from 307dfc4
workflow-compiling-service	26.31% <ø> (ø)		Carriedforward from 307dfc4

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

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[mengw15]

LGTM