#312: Update CI to run async coverage and preserve sync-only installation checks

[Mattsface]

Why

The existing CI workflow was still tied to the old release branch and installed the package without the optional async extra.

That meant the async adapter tests could be skipped in CI even though they passed locally. We also wanted to keep coverage for the normal sync-only installation so HTTPX stays optional.

What

• Updated CI triggers to run for main and release/**
• Updated the Python 3.10–3.14 test matrix to install the async extra
• Added a separate sync-only job that:
  • installs only the normal runtime dependencies
  • verifies HTTPX is not installed
  • verifies Mlb and MlbDataAdapter still import normally
• Updated the CI contract tests to reflect the new release branch and async coverage behavior
• Left the existing package build and release validation flow unchanged

Tests

Ran the full offline suite locally with the async extra installed:

poetry install --no-interaction -E async

poetry run pytest \
  tests/ \
  --ignore=tests/external_tests