Exercises without explicit m/n are dropped #775

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Overview

An ExerciseNode with no m/n fails validation and is left out of the channel. This covers the constructor default, {"mastery_model": M_OF_N} alone, and CSV exercises with blank M/N columns. The defaults that v0.7.3 filled in are applied only after validation has already failed. m=None crashes the run.

Complexity: Low
Target branch: main

Context

• nodes.py:1137-1159 checks for "m"/"n" before calling process_exercise_data() (nodes.py:1093-1108), which is what fills in the defaults.
• Error:

  InvalidNodeException: M of N mastery model is missing M value
  

• With m=None, int(None) raises TypeError, which is not caught.
• The repo's tests/testchannels/csv_channel_with_exercises loses 4 of its 5 exercises.
• Introduced by cb7ba7c (Allow ContentNode class to take a uri argument for its default file #593).

The Change

m/n should be defaulted, as in v0.7.3, before validation checks them. Values that can't be converted to an integer should fail the node, not the run.

How to Get There

Build ExerciseNode(source_id="e", title="E", license=licenses.CC_BY, questions=[q]) with no exercise_data and call process_files().

Acceptance Criteria

• csv_channel_with_exercises uploads all 5 exercises.
• An exercise with no exercise_data uploads with m = n = min(5, questions), minimum 1.
• An exercise with only m uploads with n = m, and the reverse.
• m=None or a non-numeric m fails that node, and the run continues.

AI usage

I directed the regression hunt and decided which behaviour changes were intended; Claude Code compared v0.7.3 against main, reproduced each regression with scripts, and drafted this issue.