ACT URLs with improper values should return error http header codes not 200s

[nep]

Suggest an idea for ExpressionEngine

Is your feature request related to a problem? Please describe.
Accessing an ACT URL with an improper value (number or string) should generate a proper error response code header instead of a 200 response. Here are some current examples and whether they are problematic or not:

https://hopstudios.com/?ACT=15 -- 403 (fine)
https://hopstudios.com/?ACT=15.0 -- 403 (weird but fine)
https://hopstudios.com/?ACT=1.5e1 -- 403 (weird but fine)
https://hopstudios.com/?ACT=6 -- 200 (displays message, problem)
https://hopstudios.com/?ACT=8 -- 200 (displays message, problem)
https://hopstudios.com/?ACT=15.1 -- 200 (displays nothing, problem)
https://hopstudios.com/?ACT=999 -- 200 (displays nothing, problem)
https://hopstudios.com/?ACT=sdf -- 200 (displays nothing, problem)
https://hopstudios.com/?ACT=6.2e3 -- 200 (displays nothing, problem)

Describe the solution you'd like
I think a 400 would be best, but 406 might also be okay.

https://www.semrush.com/blog/http-status-codes/

Describe alternatives you've considered
Some specific ACTs could use other values, but I think as a default, 400 is good.

Teachability, Documentation, Adoption, Migration Strategy
No documentation changes would be necessary.

[intoeetive]

#4619