Add actual mypy error messages to documentation examples

[veeceey]

Summary

This PR improves the googleability of mypy documentation by including actual error messages in the examples. When users encounter mypy errors, they can now more easily find relevant documentation by searching for the exact error message text.

Changes

Made improvements to docs/source/common_issues.rst:

1. List invariance example (line 309): Changed from "mypy will complain about this, because List is invariant" to the actual error message
2. Incompatible function argument (line 332): Changed from "# Fails" to the actual error message
3. Incompatible method override examples (lines 729 and 739): Changed from "# Error" to actual full error messages

Before and After

Before

lst = new_lst  # mypy will complain about this, because List is invariant

After

lst = new_lst  # error: Incompatible types in assignment (expression has type "list[B]", variable has type "list[A]")

Benefits

1. Better searchability: Users can search for exact error messages and find relevant documentation
2. Clearer expectations: Users understand what error mypy will actually produce
3. Easier comparison: Users can compare their errors with documented examples

Rationale

As mentioned in #2393, including actual error messages makes it much easier to Google for common mypy errors and find the relevant documentation. This is especially helpful for new users who may not know which section of the documentation addresses their specific error.

Fixes #2393