ECS: add pidMode validation for FARGATE

[cm-iwata]

ECS on FARGATE only allows task for pidMode
https://docs.aws.amazon.com/AmazonECS/latest/userguide/task_definition_parameters.html

[bblommers]

Hi @cm-iwata! Looks like the linter is failing: https://github.com/getmoto/moto/actions/runs/6222529602/job/16886668644

[cm-iwata]

@bblommers
Fixed lint error that relate this PR.
I'm still getting a lint error, but this is not related to this PR.

[codecov]

Codecov Report

Merging #6825 (cf3b9ae) into master (fb9023a) will decrease coverage by 0.01%.
Report is 12 commits behind head on master.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6825      +/-   ##
==========================================
- Coverage   96.20%   96.20%   -0.01%     
==========================================
  Files         816      816              
  Lines       80034    80045      +11     
==========================================
+ Hits        76998    77007       +9     
- Misses       3036     3038       +2     

Flag	Coverage Δ
servertests	37.01% <0.00%> (-0.01%)	⬇️
unittests	96.14% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
moto/ecs/models.py	93.84% <100.00%> (+0.01%)	⬆️

... and 2 files with indirect coverage changes

[bblommers]

Awesome - thank you!

[github-actions]

This is now part of moto >= 4.2.4.dev12