Propagate part-ref caller through NodeChildrenDisplayCommand

[johnml1135]

Summary

Extracted from a rebase hunk found on the Avalonia migration branch (#964) — pulled out because it changes shared, legacy-reachable rendering behavior and shouldn't ride along in that PR. Opening this as a draft for team discussion, not as a confident fix.

XmlViewsUtils.GetDisplayCommandForColumn1's "part" case builds a NodeChildrenDisplayCommand(part) and always passes a null caller into ProcessChildren. XmlVc's own "part" handling passes a non-null caller in the equivalent situation. This change makes the two agree by carrying the part-ref node down as the caller.

There's a pre-existing // Review JohnT: comment sitting right on this code in XmlViewsUtils.cs (removed by this diff) that names exactly this gap and says it couldn't be confirmed to matter:

In XmlVc, "part" is the one thing that calls ProcessChildren with non-null caller. this should make some difference here, but I can't figure what yet, or come up with a test that fails.

Uncertainty — please weigh in

• Is this actually needed on main today? The rebase hunk's comment referenced "the 'Grammatical Info.' blank column bug in the Avalonia browse table" with an unfilled LT- ticket reference — I don't have a real ticket number and haven't reproduced a blank-column bug on main myself. I can't confirm this fixes a live bug vs. a bug that only manifests once the Avalonia browse-column path exists.
• No test added. The original JohnT comment says he couldn't construct a failing test either. If we land this, it should come with a regression test for whatever blank-column scenario motivated it — otherwise we're changing shared rendering code on faith.
• Verified: builds clean (XMLViews.csproj, 0 errors/warnings) and no other call site depends on the old single-arg-only shape.

Test plan

• Confirm/attach the real ticket (or a repro) for the blank-column symptom this addresses
• Add a regression test for a "part"-based browse column that reads a caller-relative attribute (e.g. ws)
• Manual smoke test of existing "part"-based browse columns to confirm no visual/behavioral change for the common case

🤖 Generated with Claude Code

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This change is 

[github-actions]

NUnit Tests

    1 files  ±0      1 suites  ±0   10m 42s ⏱️ + 3m 52s
4 303 tests +1  4 230 ✅ +1  73 💤 ±0  0 ❌ ±0 
4 312 runs  +1  4 239 ✅ +1  73 💤 ±0  0 ❌ ±0 

Results for commit f4b3f76. ± Comparison against base commit f505729.

♻️ This comment has been updated with latest results.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 46.15385% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.98%. Comparing base (f505729) to head (f4b3f76).

Files with missing lines	Patch %	Lines
Src/Common/Controls/XMLViews/XmlVc.cs	41.66%	4 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1009   +/-   ##
=======================================
  Coverage   32.97%   32.98%           
=======================================
  Files        1202     1202           
  Lines      278291   278299    +8     
  Branches    37166    37168    +2     
=======================================
+ Hits        91776    91786   +10     
+ Misses     158649   158647    -2     
  Partials    27866    27866           

Files with missing lines	Coverage Δ
Src/Common/Controls/XMLViews/XmlViewsUtils.cs	41.44% <100.00%> (+0.58%)	⬆️
Src/Common/Controls/XMLViews/XmlVc.cs	27.99% <41.66%> (+0.08%)	⬆️

... and 1 file with indirect coverage changes

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