fix(pytest): surface collection errors instead of 'No tests collected'

[mariofeter]

Problem

The pytest summarizer returns Pytest: No tests collected whenever the summary line contains no passed/failed/skipped counts. This hides:

• Collection errors (ImportError/SyntaxError in a test module): pytest prints an ERRORS section and 1 error in 0.06s, exits 2 — rtk reports "No tests collected" and the root cause is lost. This is exactly the RED case TDD workflows rely on.
• Non-session output: rtk pytest --version also reports "No tests collected" instead of the version string.

Repro (pytest 9, real output fixture included):

$ rtk pytest tests/        # tests/test_broken.py has a broken import
Pytest: No tests collected   # before — root cause invisible

Fix

• parse_summary_line now parses N error(s) counts (both === 1 error in 0.12s === and bare -q 1 error in 0.06s; the bare form requires a leading integer so traceback prose can't masquerade as a summary line)
• the ERRORS section is treated like FAILURES, so collection tracebacks are captured
• ERROR entries render under an Errors: header with an [ERR] tag
• root-cause lines (> / E markers) are selected before traceback location boilerplate within the existing 3-line cap, so E ModuleNotFoundError: ... always survives
• unrecognized non-session output (--version, --help, plugin banners) is passed through unchanged; "No tests collected" now requires pytest to have actually said so (no tests ran / collected 0 items)
• !!! Interrupted !!! banners are skipped in failure details (the error count already lands in the header)
• dropped the dead test_files collection (was already unused: _test_files)

After

$ rtk pytest tests/
Pytest: 0 passed, 0 failed, 1 error

Errors:
1. [ERR] collecting tests/test_broken.py
     ImportError while importing test module '/tmp/proj/tests/test_broken.py'.
     /usr/lib/python3.14/importlib/__init__.py:88: in import_module
     E   ModuleNotFoundError: No module named 'nonexistent_module'

$ rtk pytest --version
pytest 9.0.3

Green runs are unchanged (Pytest: N passed), normal assertion failures unchanged.

Test plan

• New unit tests: quiet-mode collection error (real pytest 9 fixture), banner-mode collection error, --version passthrough, parse_summary_line error counts
• All existing pytest filter tests pass (24 total)
• cargo fmt --all && cargo clippy --all-targets clean
• Manual: collection error (exit 2 propagated), green suite, mixed pass/fail, --version

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