fix(log4net): send event properties as log attributes on log4net 3.x

[jamescrosswell]

Closes #5557

Summary

On log4net 3.x, SentryAppender silently dropped every event property from structured logs — no property.* attribute was ever attached to a SentryLog, with no exception and no warning.

log4net.Util.PropertiesDictionary gained IDictionary<string, object?> in 3.x, and that changed what its non-generic IEnumerable yields: KeyValuePair<string, object?> instead of DictionaryEntry. Sentry.Log4Net is compiled against its 2.0.12 floor and walks exactly that non-generic enumerator, so property is DictionaryEntry matched nothing and the loop body never ran.

The fix reads through GetKeys() + the indexer, the way the sibling SentryEvent path in GetLoggingEventProperties already does. That API behaves identically on both majors, so the appender is version-agnostic and the package floor stays at 2.0.12 — no new minimum for consumers still on log4net 2.x.

Notes for review

• New test project, Sentry.Log4Net.V3.Tests. This is a runtime-only difference — nothing that builds against 2.x can catch it — so covering it needs the appender actually loaded against a 3.x assembly. The new project links the same two appender test files (Compile Include, not copies, so the two majors can't drift) and references log4net 3.4.0, while Sentry.Log4Net.Tests stays on 2.0.15. Both majors now run in CI. I confirmed the new project has teeth: reverting just the appender change reproduces exactly the three failures from the issue (52 passed / 3 failed), and both suites are green with it.

• The two Verify-based tests are deliberately not linked into the new project — their snapshots are keyed off the original file paths, so running them from two projects would have both writing the same files.

• Three InternalsVisibleTo entries were needed for the new assembly (Sentry, Sentry.Log4Net, Sentry.Testing), following the pattern every other test project uses. These don't appear in the API approval snapshots, and ApiApprovalTests is green.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.76%. Comparing base (0004ef8) to head (bd94d96).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5559   +/-   ##
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  Coverage   74.75%   74.76%           
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  Files         515      515           
  Lines       18963    18963           
  Branches     3694     3694           
=======================================
+ Hits        14176    14177    +1     
+ Misses       3909     3907    -2     
- Partials      878      879    +1     

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