Update generic associations examples for strict typing

Update generic associations examples for strict typing

### Description

Following previous work in #10450 and #12031, add more type hints to the examples.

I added a new test case to exercise these examples in the future as well.

### Checklist

This pull request is:

- [x] A documentation / typographical / small typing error fix
	- Good to go, no issue or tests are needed

**Have a nice day!**

Closes: #13082
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/13082
Pull-request-sha: 7a2bed3f6d9fe33c621ac2a6ca56364fa5fe7053

Change-Id: Ia143abbc4c8491c2976203d1b99162652b26b417