StepFunctions: fix apigateway:invoke task parameters being dropped

[Smelton01]

Overview

Closes #10163

The parser-based StepFunctions implementation only accepted ApiEndpoint and Method for apigateway:invoke tasks, rejecting all other documented parameters.

This PR:

• Accepts the optional request params Headers, Stage, Path, QueryParameters, RequestBody, AllowNullValues, and AuthType according to the docs
• Preserves multi-value query parameters and headers instead of dropping all but the last value
• Fixes a KeyError when the API response has no Content-Length, and a TypeError (FailureEvent missing env) that masked real task failures

Added integration tests for the success and failure paths.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.37%. Comparing base (fd49aae) to head (e51bb8e).
⚠️ Report is 38 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10164      +/-   ##
==========================================
+ Coverage   93.24%   93.37%   +0.12%     
==========================================
  Files        1326     1336      +10     
  Lines      121003   121816     +813     
==========================================
+ Hits       112831   113740     +909     
+ Misses       8172     8076      -96     

Flag	Coverage Δ
servertests	27.87% <0.00%> (-0.09%)	⬇️
unittests	93.34% <100.00%> (+0.12%)	⬆️

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