Move collection error abort from pytest_runtestloop to pytest_collection - #15384

Closes #15028.

The check that stops the session after collection errors lived in the default pytest_runtestloop. Since that hook is firstresult, a plugin that implements its own pytest_runtestloop never reached the check, and tests ran even though collection failed. As suggested by @bluetech in the issue, this moves the check into the default pytest_collection.

This picks up #15061 by @gyanu2507, which the author closed and invited others to continue. Compared to that PR:

• In the pytest_collection docstring, Interrupted is written as a literal instead of :class:~pytest.Interrupted. Interrupted is not exported from the pytest namespace, so I believe that reference is what failed the Read the Docs build there (the docs build is nitpicky).
• Added a second test checking that with --continue-on-collection-errors, a plugin's pytest_runtestloop still runs.

Changes:

• src/_pytest/main.py: moved the testsfailed check from pytest_runtestloop to pytest_collection.
• src/_pytest/hookspec.py: updated the pytest_collection and pytest_runtestloop docstrings.
• testing/test_collection.py: two new tests.
• changelog/15028.breaking.rst: breaking change entry, since the current behavior is documented.

How I tested (Windows, Python 3.12):

• Reproduced the issue with a small project whose conftest.py replaces pytest_runtestloop and a test file that fails to import. Before the change: 1 passed, 1 error. After: Interrupted: 1 error during collection.

• With the original main.py, the first new test fails (INTERNAL_ERROR instead of INTERRUPTED), and it passes with the change.

• testing/test_collection.py: 109 passed. test_main.py, test_terminal.py, acceptance_test.py pass except test_pydoc, which also fails on main on my machine (UnicodeDecodeError, a Windows encoding issue unrelated to this change).

• pre-commit passes on the changed files.

• Include new tests or update existing tests when applicable.

• Allow maintainers to push and squash when merging my commits.

• Add text like closes #XYZW to the PR description and/or commits.

• If AI agents were used, they are credited in Co-authored-by commit trailers.

• Create a new changelog file in the changelog directory.

• Add yourself to AUTHORS in alphabetical order.