feat: enable ListSignal in generator-plugin-signals

[taefi]

Description

This enables the generator-plugin-signals to correctly generate TS services for the endpoints with methods returning instances of ListSignal.

Follow-up for #2775
(Based on #2775)

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

[sonarcloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
2.6% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.52%. Comparing base (ecd0355) to head (c67de25).

Additional details and impacted files

@@                      Coverage Diff                       @@
##           taefi/feat/add-list-signal    #2810      +/-   ##
==============================================================
+ Coverage                       92.51%   92.52%   +0.01%     
==============================================================
  Files                              83       83              
  Lines                            2765     2769       +4     
  Branches                          715      716       +1     
==============================================================
+ Hits                             2558     2562       +4     
  Misses                            156      156              
  Partials                           51       51              

Flag	Coverage Δ
unittests	92.52% <100.00%> (+0.01%)	⬆️

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