add http3 fingerprint tests

[cornmander]

Summary

Refs #161.

• add an aioquic-backed local HTTP/3 test server fixture
• add HTTP/3 / QUIC fingerprint checks for chrome145, chrome146, and firefox147
• store expected H3 headers, H3 SETTINGS, and QUIC transport parameters in tests/http3_targets.yaml
• document the local Docker test path for running the focused HTTP/3 tests

Validation

• python3 -m py_compile tests/test_impersonate.py
• /tmp/codex-aioquic-inspect/bin/python -m pytest --collect-only -q test_impersonate.py -k http3
• git diff --check
• docker build -t curl-impersonate -f docker/debian.dockerfile .
• docker build -t curl-impersonate-tests tests/
• docker run --rm curl-impersonate-tests -k http3 --log-cli-level DEBUG

Docker testing

The focused HTTP/3 tests can be run locally with:

docker build -t curl-impersonate -f docker/debian.dockerfile .
docker build -t curl-impersonate-tests tests/
docker run --rm curl-impersonate-tests -k http3 --log-cli-level DEBUG

The Docker flow was run locally after creating the PR. It initially caught a Chrome 146 QUIC fixture mismatch and duplicate curl output-option warning; both are fixed in the latest push.