fix(core): keep run() from raising on invalid UTF-8 exercises

[Huoyanlifusu]

Summary

Fixes #72

run() documents a no-raise contract: the CLI and TUI rely on receiving a structured RunResult. An invalidly encoded learner exercise currently escapes that boundary as UnicodeDecodeError from read_text().

Catch the decode error and return a failing RunResult with an actionable stderr message instead.

Tests

(.venv) yy@yy-desktop:~/opensource/pythonlings$ python -m pytest tests/unit/test_runner.py tests/integration/test_cli_run.py tests/integration/test_cli_verify.py -q
python -m pytest -q
pythonlings --root tests/fixtures/passing_curriculum verify
..........................                                                                                                         [100%]
26 passed in 1.93s
.................................................................................................................................. [ 60%]
......................................................................................                                             [100%]
216 passed in 26.43s
✓ passing1
✓ passing2

Screenshots

Checklist

• Updated docs when behavior changed
• Added or updated tests
• Verified python -m pytest -q

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[abhiksark]

Thanks for tackling this — the run() change is going in the right direction, and the current test suite is green. I found one blocker before we can merge it.

An invalid UTF-8 exercise still crashes the TUI on startup before run() gets a chance to return the new RunResult. I reproduced it with an invalid a1.py: TrackScreen._load_current() calls EditorPane.load_exercise(), which decodes the file directly and raises UnicodeDecodeError. The output panel also reads the exercise directly when building the goal text. The app stops instead of showing the failure through its normal output path, which is one of #72’s acceptance requirements.

Could you handle that TUI load path and add a regression test showing the app stays open and displays the encoding error? The runner error should also include the exercise name or path; right now it only says exercise is not valid UTF-8, so it does not identify which exercise failed.

Validation on the current branch:

• targeted runner/CLI tests — 26 passed
• full suite — 216 passed
• passing curriculum verification — passed
• manual CLI check — exits cleanly without a traceback
• manual TUI check — still raises UnicodeDecodeError and stops the app

[abhiksark]

The core runner fix works, but #72 is not fully satisfied yet. Invalid UTF-8 still raises from EditorPane.load_exercise() before the TUI reaches run(), so the app stops instead of rendering a normal failure. Please cover the TUI read path with regression coverage, and include the affected exercise name or path in the runner error.