Fix incorrect “forgot new” errors caused by operator-like specifiers

[lola831]

Description

This PR fixes the parser issue where valid expressions like x in [0] or complex behavior conditions were incorrectly matched by invalid_scenic_instance_creation, causing Scenic to raise a “Perhaps you forgot new?” error at the wrong line. The problem was that operator-like specifiers (in, at, offset by, offset along) were treated the same as normal Scenic instance specifiers. I split these into two groups and restricted the “forgot new” rule to non-operator specifiers only. This resolves both #345 and #356: valid expressions are no longer misinterpreted, and incorrect instance specifiers are now reported at the correct location.

Issue Link

# 345
# 356

Checklist

• I have tested the changes locally via pytest and/or other means
• I have added or updated relevant documentation
• I have autoformatted the code with black and isort
• I have added test cases (if applicable)

Additional Notes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.88%. Comparing base (f0e3e60) to head (91a38fb).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #426      +/-   ##
==========================================
- Coverage   89.93%   89.88%   -0.06%     
==========================================
  Files          54       54              
  Lines       13567    13560       -7     
==========================================
- Hits        12202    12188      -14     
- Misses       1365     1372       +7     

see 6 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[dfremont]

Nice fix, thanks Lola!