chore: replace `respx` mocks with `pytest-httpserver` in tests #324

Job	Run time
Async dostrings check / Async docstrings check	18s
Unit tests / Unit tests (ubuntu-latest, 3.11)	40s
Lint check / Lint check (3.13)	9s
Lint check / Lint check (3.10)	12s
Lint check / Lint check (3.9)	8s
Unit tests / Unit tests (windows-latest, 3.12)	1m 2s
Lint check / Lint check (3.12)	10s
Type check / Type check (3.10)	17s
Lint check / Lint check (3.11)	13s
Unit tests / Unit tests (windows-latest, 3.10)	55s
Type check / Type check (3.9)	17s
Unit tests / Unit tests (windows-latest, 3.11)	1m 4s
Unit tests / Unit tests (ubuntu-latest, 3.13)	37s
Unit tests / Unit tests (ubuntu-latest, 3.9)	41s
Type check / Type check (3.13)	17s
Type check / Type check (3.12)	20s
Unit tests / Unit tests (windows-latest, 3.13)	1m 0s
Type check / Type check (3.11)	16s
Unit tests / Unit tests (ubuntu-latest, 3.10)	38s
Unit tests / Unit tests (windows-latest, 3.9)	53s
Docs check / Docs check	1m 14s
Unit tests / Unit tests (ubuntu-latest, 3.12)	40s
Integration tests / Approve integration tests for PRs from forks	3s
Integration tests / Integration tests	-1s
Integration tests / Integration tests for PRs from forks	0s
	12m 3s

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