feat: add contains() operation to QuerySet

[seladb]

Add QuerySet.contains() method for checking if object exists in queryset

Description

Added QuerySet.contains() which checks if a specific model instance exists within a queryset. Implementation includes:

• QuerySet.contains(obj) method in tortoise/queryset.py that returns a ContainsQuery object
• ContainsQuery class that extends ExistsQuery and adds filtering by the object's primary key

Motivation and Context

This feature enables checking if a specific model instance belongs to a filtered queryset, useful for membership checks without fetching the full object.

It's also in par with Django API.

How Has This Been Tested?

• Added test_contains in tests/test_queryset.py covering all cases
• All existing tests pass

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing seladb:add-contains (f0e7e77) with develop (2df46d0)}

[seladb]

@abondar MSSQL tests fail randomly, but shouldn't be related to the changes in this PR