fix(gate): make __typename returns the variant name on unions

[Natoandro]

• Add missing implementation for static injection for parameter transformations on the typegate
• Solves MET-642: Fix the __typename result on union variants: return the variant name instead of the parent type name

Migration notes

N/A

Checklist

• The change comes with new or modified tests
• Hard-to-understand functions have explanatory comments
• End-user documentation is updated to reflect the change

[linear]

MET-642 (gate) `__typename` on union selections should hold member title

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.

Project coverage is 75.75%. Comparing base (67c1d0f) to head (aeb84e7).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...pegate/src/engine/planner/parameter_transformer.ts	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #838      +/-   ##
==========================================
- Coverage   75.95%   75.75%   -0.21%     
==========================================
  Files         144      144              
  Lines       16696    16714      +18     
  Branches     1639     1637       -2     
==========================================
- Hits        12682    12662      -20     
- Misses       3991     4029      +38     
  Partials       23       23              

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[j03-dev]

LGTM