feat: log4net appender no longer initializes the SDK

[jamescrosswell]

The log4net portion of #5245, stacked on #5585 (NLog) and following the same design. The Sentry appender for log4net now only sends log events to Sentry; Sentry has to be initialized separately via SentrySdk.Init, UseSentry, etc.

Part of #5245

Breaking changes

• SentryAppender.Dsn is removed, along with the appender initializing the SDK on the first append.
• SentryAppender.Environment is removed. Events and structured logs take the environment from the options used to initialize Sentry.
• SentryAppender no longer overrides OnClose. It only existed to dispose the SDK the appender had initialized, so closing the log4net repository no longer flushes Sentry; disposing the handle from SentrySdk.Init does.
• XML configs that still set <Dsn> or <Environment> on the appender keep loading. log4net reports each one through its internal logging (log4net:ERROR XmlHierarchyConfigurator: Cannot find Property [Dsn] to set object on [Sentry.Log4Net.SentryAppender]) and applies the remaining properties, but Sentry is no longer initialized.

Before:

<appender name="SentryAppender" type="Sentry.Log4Net.SentryAppender, Sentry.Log4Net">
  <Dsn value="https://key@sentry.io/1" />
  <Environment value="dev" />
</appender>

After:

using var _ = SentrySdk.Init(o =>
{
    o.Dsn = "https://key@sentry.io/1";
    o.Environment = "dev";
});

<appender name="SentryAppender" type="Sentry.Log4Net.SentryAppender, Sentry.Log4Net" />

Notes for review

• On a disabled hub the appender now returns immediately. That's the same path the old code took when no Dsn was set.
• The unit test fixture now uses an enabled hub. Previously the substitute hub reported IsEnabled == false, and events were only captured because the mocked init had run.
• Like NLog, log4net needs no UseLog4Net(): identity, properties and the SDK name are all applied by the appender to the events and logs it creates.
• SendIdentity and MinimumEventLevel stay; they configure what the appender sends, not the SDK.
• ApiApprovalTests.Run.Net4_8 and DotNet10_0 are copies of the regenerated DotNet11_0 snapshot, which all three matched byte-for-byte beforehand. Net4_8 can't regenerate on macOS, and DotNet10_0 isn't run at all since the test project only targets the latest TFM.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.56%. Comparing base (489d7ce) to head (6438db5).

Additional details and impacted files

@@                           Coverage Diff                           @@
##           feat/no-init-from-logging-nlog-5245    #5592      +/-   ##
=======================================================================
- Coverage                                74.58%   74.56%   -0.03%     
=======================================================================
  Files                                      514      514              
  Lines                                    18745    18728      -17     
  Branches                                  3646     3641       -5     
=======================================================================
- Hits                                     13981    13964      -17     
  Misses                                    3891     3891              
  Partials                                   873      873              

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