fix(capa): reject empty submissions with a clear message

[kingoftech-v01]

Description

Submitting a CAPA problem with no answer selected returned a cryptic, fragile error message to the learner instead of a clear "you must provide an answer" hint. The underlying flow had three defects:

1. No server-side empty-answer validation.
2. grade_answers({}) raised a cryptic StudentInputError.
3. The error message was passed through a codejail-traceback parser that expects a specific \n-separated format and crashes / falls back to the raw message on single-line errors.

User impact (Learner): Learners who click Submit without selecting an answer now see "You must provide an answer before submitting." and are not charged an attempt.

Root Cause

• xmodule/capa_block.py:1753 calls make_dict_of_responses(data) on whatever POST body comes in. No validation.
• xmodule/capa_block.py:1814 sends the empty dict to self.lcp.grade_answers(answers), which raises StudentInputError.
• xmodule/capa_block.py:1845-1850 runs full_error.split("\\n")[-2].split(": ", 1)[1]. For a single-line error the split yields [full_error], [-2] raises IndexError, and the handler falls back to msg = full_error — exposing the raw, technical English message via gentle_alert in the frontend.

The fragile parser was added in commit b9e768410f (2017-02-01, Peter Pinch @ MIT, "return only error value to students when codejail raises an exception"). It was designed to handle codejail tracebacks containing a \n separator — not ordinary StudentInputError messages. Empty-answer handling was never considered.

Fix

Single, surgical early-return in capa_block.py just before self.lcp.grade_answers(...). If answers is empty or every value is blank ("", [], {}, None), return a clear translatable message without incrementing the attempt counter. Publish a problem_check_fail tracking event with failure="missing_answer" for observability parity with the existing pre-grading guards (closed, unreset, wait-time).

The predicate deliberately does NOT match 0 or False (valid numeric/boolean answers) and does NOT match file uploads (which are file objects, not containers).

The fragile codejail-traceback parser at lines 1845-1850 is intentionally left alone — it is still correct for genuine codejail tracebacks and changing it risks regressing test_submit_problem_error_with_codejail_exception.

Supporting Information

• Issue: [Test failure] <TC_LEARNER_45>: <Incorrect error message displays incase of Clicking on Submit button on courses screen without adding answers> #36829
• Parser origin: commit b9e768410f (Peter Pinch, 2017-02-01)
• Later touched by 9ee7ef1acf (reduce escaping, 2018) and file moves (b0d41f6ebb, 4c005e86e8)

Testing Instructions

1. Open any CAPA problem in the LMS (multiple choice, checkbox, text input).
2. Click Submit without selecting any answer.
3. Before the fix: a cryptic technical message is shown, or the problem is marked incorrect.
4. After the fix: a clear "You must provide an answer before submitting." alert is shown, and the attempt counter is NOT incremented.
5. Verify normal answered submissions still work correctly (regression check).

Four new tests in xmodule/tests/test_capa_block.py:

• test_unit_submit_problem_empty_answer — Unit: empty dict rejection
• test_integration_submit_problem_all_blank_values — Integration: all blank values path
• test_submit_problem_zero_is_not_empty — Regression guard: "0" is a valid answer
• test_bug_36829_regression_empty_answer_publishes_fail_event — Regression: tracking event emission

Deadline

None

Other Information

• No database migrations
• No new dependencies
• No deprecations
• No schema/API change — the submit_problem return shape is already used by the other pre-grading guards
• New translatable string ("You must provide an answer before submitting.") follows the same style as existing sibling strings ("Problem is closed.", "You must wait at least...") and will be picked up automatically by manage.py makemessages
• Client-side guard in display.js:932-1001 has a hardcoded English "Select an option" check that is orthogonal to this bug and should be addressed in a separate UI-focused PR
• Safe to backport (teak label on the issue)

Closes #36829

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