Fix Compare-InstallerSnapshots.ps1 StrictMode crash on empty UninstallEntries

[johnml1135]

Compare-InstallerSnapshots.ps1 crashed under Set-StrictMode whenever a
before/after snapshot had zero UninstallEntries. Collect-InstallerSnapshot
serializes an empty list as {} (a Windows PowerShell ConvertTo-Json
quirk), and the script read .DisplayName directly off that property-less
object, which throws under strict mode. This surfaced while gathering
installer evidence: a silent MSI install whose before-snapshot legitimately
had 0 uninstall entries made the diff tool abort instead of producing a
report.

The fix extracts DisplayName through a new Get-DisplayNameList helper
that tolerates $null, a single unwrapped object, and the empty {}
object, skipping any entry without a usable DisplayName. Everything else
about the diff -- registry and file comparison -- is untouched.

Where to look

• Get-DisplayNameList (scripts/Agent/Compare-InstallerSnapshots.ps1) --
  the null/empty/unwrapped-object handling is the whole fix.
• Both call sites (before- and after-snapshot UninstallEntries) now go
  through the same helper, so the two snapshots can't drift in how they
  handle a missing entry.

Deliberately not here

• Collect-InstallerSnapshot's {}-for-empty-list serialization is
  unchanged; hardening the producer to always emit [] is a separate,
  reasonable follow-up.

Verification

• Ran the script against a synthetic before/after pair with an empty {}
UninstallEntries (the exact shape that crashed): exits 0, produces the
  expected diff.
• gitlint --ignore body-is-missing --commits origin/main..HEAD -- clean.
• All 6 CI checks pass.

---

Reading this a year from now -- start here

The one thing worth knowing beyond the pitch above: the {}-for-empty-list
quirk lives in Collect-InstallerSnapshot's JSON serialization, not in this
script. This script tolerates it rather than fixing the producer, so any
other consumer of a snapshot file needs the same tolerance.

Preflight review details

See .review/summary.md on the reviewing machine for the full pass:
no contract/API impact, no open findings, verification steps as listed
above.

---

This change is 

[github-actions]

NUnit Tests

    1 files   -     1      1 suites   - 1   9m 28s ⏱️ - 5m 3s
5 761 tests ±    0  5 680 ✅ +   38  81 💤 ± 0  0 ❌  - 38 
5 770 runs   - 5 770  5 689 ✅  - 5 615  81 💤  - 81  0 ❌  - 74 

Results for commit 1051486. ± Comparison against base commit 7f93348.

♻️ This comment has been updated with latest results.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 40.33%. Comparing base (f505729) to head (1051486).
⚠️ Report is 41 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1008      +/-   ##
==========================================
+ Coverage   32.97%   40.33%   +7.35%     
==========================================
  Files        1202     1542     +340     
  Lines      278291   365073   +86782     
  Branches    37166    40233    +3067     
==========================================
+ Hits        91776   147237   +55461     
- Misses     158649   188551   +29902     
- Partials    27866    29285    +1419     

see 501 files with indirect coverage changes

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