chore(frontend): remove the unused scroll-to-step request stream

[aglinxinyuan]

What changes were proposed in this PR?

Removes AgentService's scroll-to-step request stream, which can never fire. Pure deletion, no behaviour change: −74 lines.

The chain exists end to end but has no trigger: requestScrollToStep() is the only producer and nothing calls it, so scrollToStepSubject never emits, the agent-chat subscriber never runs, and the private scrollToStep() it dispatches to is unreachable.

(no caller)  ->  requestScrollToStep()  ->  scrollToStepSubject  ->  agent-chat subscriber  ->  scrollToStep()
     ^ the chain starts here and nothing ever enters it

History

Introduced by	#4495 (2026-04-27) — "feat(agent-service): add agent-service that manages LLM agents", which shipped the whole chain at once
Usage removed by	never wired up — no file outside agent.service.ts has called requestScrollToStep at any point in the history, and no UI control invokes it

Reviewer note: the sibling showPortShapes$ stream in the same "canvas annotation state" block is live and is untouched; only the scroll-to-step members and the section comment's mention of them change. scrollToMessage and setHoveredMessage, which the dead handler called, both keep other callers.

Any related issues, documentation, discussions?

Closes #8610

How was this PR tested?

Existing tests only — this PR removes code and the tests that covered it.

From frontend/:

• npx ng test --watch=false --include='**/agent.service.spec.ts' --include='**/agent-chat.component.spec.ts' --include='**/agent-panel.component.spec.ts' — 193 tests pass across the three files.
• yarn --cwd frontend format:ci — clean.

Verification, re-runnable by a reviewer:

git grep -nF 'scrollToStep$'     # use -F: a trailing $ is a regex anchor
git grep -n requestScrollToStep

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Automated Reviewer Suggestions

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

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

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.77%. Comparing base (957b6c9) to head (53b65a5).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8613      +/-   ##
============================================
- Coverage     92.78%   92.77%   -0.01%     
  Complexity     4902     4902              
============================================
  Files          1236     1236              
  Lines         52165    52155      -10     
  Branches       6411     6409       -2     
============================================
- Hits          48402    48389      -13     
- Misses         2191     2196       +5     
+ Partials       1572     1570       -2     

Flag	Coverage Δ		*Carryforward flag
access-control-service	71.78% <ø> (ø)		Carriedforward from 957b6c9
agent-service	99.16% <ø> (ø)		Carriedforward from 957b6c9
amber	88.63% <ø> (ø)		Carriedforward from 957b6c9
computing-unit-managing-service	55.20% <ø> (ø)		Carriedforward from 957b6c9
config-service	87.37% <ø> (ø)		Carriedforward from 957b6c9
file-service	81.53% <ø> (ø)		Carriedforward from 957b6c9
frontend	96.57% <ø> (-0.02%)	⬇️
notebook-migration-service	83.73% <ø> (ø)		Carriedforward from 957b6c9
pyamber	98.48% <ø> (ø)		Carriedforward from 957b6c9
workflow-compiling-service	74.09% <ø> (ø)		Carriedforward from 957b6c9

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

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