Fix incorrect parameter name Tw_out -> Twall_out in validator message

[engineer-scientist]

Description

The isothermal-out wall-temperature positivity check in
toolchain/mfc/case_validator.py printed a parameter name that does not
exist. The value is correctly fetched as bc_{dir}%Twall_out, but the
error string in the positivity prohibit call referred to
bc_{dir}%Tw_out. The actual parameter is Twall_out (defined in
toolchain/mfc/params/definitions.py); there is no Tw_out parameter.
A user setting a non-positive exit wall temperature was told to fix a
parameter that doesn't exist, misdirecting the fix.

This changes the message string from bc_{dir}%Tw_out to
bc_{dir}%Twall_out, matching the parallel Twall_in branch. The change
is diagnostic-message-only — no validation logic is affected, and it is
output-neutral / golden-file-safe.

Closes #1480.

Type of change

• Bug fix

Testing

This is a diagnostic-string-only change with no effect on control flow or
numerical output, so no case results change. I verified it by:

• Confirming Tw_out appeared exactly once in the codebase (the incorrect
  message) and that Twall_out is the parameter actually defined in
  toolchain/mfc/params/definitions.py.
• Reviewing git diff to confirm the change is a single line, with no
  unrelated or line-ending changes.

Checklist

Check these like this [x] to indicate which of the below applies.

• I added or updated tests for new behavior
• I updated documentation if user-facing behavior changed

Neither applies: no behavior changes (message text only), and no
user-facing documentation references this string.

See the developer guide for full coding standards.

GPU changes (expand if you modified src/simulation/)

- [ ] GPU results match CPU results - [ ] Tested on NVIDIA GPU or AMD GPU

No changes to src/simulation/; GPU section not applicable.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.62%. Comparing base (30df8e5) to head (c75ae2d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1666   +/-   ##
=======================================
  Coverage   59.62%   59.62%           
=======================================
  Files          83       83           
  Lines       21246    21246           
  Branches     3145     3145           
=======================================
  Hits        12668    12668           
  Misses       6467     6467           
  Partials     2111     2111           

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