Convert simpler tests to assert_type() tests

[intgr]

I tried to convert existing tests with minimal changes -- such that in many cases "git blame" can still find the commit where the original .yml tests were added.

Tests written in regular .py files using assert_type() have lots of advantages compared to pytest-mypy-plugins .yml-format tests:

• MUCH faster to run
• Easier to write/maintain: autoformatting, auto-fixes provided by Ruff. IDE allows auto-adding imports, etc.
• Supported by multiple typecheckers, not just mypy (though there is no plan to run other typecheckers for now)

However, not all tests can be converted -- pytest-mypy-plugins tests are still useful for more complex cases:

• Tests using multiple files (e.g. separate models.py)
• Tests checking that mypy correctly raises certain error messages.
• Parameterized tests.

[intgr]

We still typecheck with Python 3.10, which doesn't have typing.assert_type

[Copilot]

Pull Request Overview

This PR converts simpler typecheck tests from pytest-mypy-plugins .yml format to regular Python files using assert_type(). The conversion maintains test coverage while improving test performance, maintainability, and developer experience through IDE support and autoformatting.

Key changes:

• Migrated 50+ test cases from .yml files to .py files using assert_type()
• Retained complex tests (multi-file, error message validation, parameterized) in .yml format
• Added mypy validation of test files to CI workflow

Reviewed Changes

Copilot reviewed 27 out of 28 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/typecheck/test_views.yml	Removed 5 simple test cases (e.g., request types, generics)
tests/typecheck/test_test.yml	Removed entire file containing client API test cases
tests/typecheck/test_serializers.yml	Removed 10 test cases for serializer functionality
tests/typecheck/test_routers.yml	Removed entire file containing router URL tests
tests/typecheck/test_request.yml	Removed entire file containing request querydict tests
tests/typecheck/test_pagination.yml	Removed entire file containing pagination tests
tests/typecheck/test_mixins.yml	Removed entire file containing mixin tests
tests/typecheck/test_filters.yml	Removed entire file containing filter backend tests
tests/typecheck/test_fields.yml	Removed 2 test cases for field functionality
tests/typecheck/test_exceptions.yml	Removed 3 test cases for exception declarations
tests/typecheck/test_decorators.yml	Removed 4 test cases, kept error validation tests
tests/typecheck/test_compat.yml	Removed entire file containing compatibility tests
tests/typecheck/test_api_client.yml	Removed entire file containing API client tests
tests/assert_type/views.py	Added converted view-related tests
tests/assert_type/test.py	Added converted test client tests
tests/assert_type/serializers.py	Added converted serializer tests
tests/assert_type/routers.py	Added converted router tests
tests/assert_type/request.py	Added converted request tests
tests/assert_type/pagination.py	Added converted pagination tests
tests/assert_type/mixins.py	Added converted mixin tests
tests/assert_type/filters.py	Added converted filter tests
tests/assert_type/fields.py	Added converted field tests
tests/assert_type/exceptons.py	Added converted exception tests (note: filename typo)
tests/assert_type/decorators.py	Added converted decorator tests
tests/assert_type/compat.py	Added converted compatibility tests
pyproject.toml	Added linter exceptions and banned API for assert_type tests
.github/workflows/test.yml	Added mypy validation step for test files

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[sobolevn]

I am ok with this, but I didn't review all the test changes, I hope that they are the same :)