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

• The appender no longer initializes the SDK on the first append. SentryAppender.Dsn remains as a tombstone — [Obsolete(..., error: true)] with a setter that throws NotSupportedException carrying migration guidance, matching feat(serilog): configuring a DSN on the sink now fails with a migration error #5611 (Serilog) and feat: NLog target no longer initializes the SDK #5585 (NLog).
• 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.

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" />

The tombstone behaves differently here than in Serilog and NLog

log4net cannot be made to fail configuration loading. XmlHierarchyConfigurator.SetParameter catches exceptions thrown while setting a parameter, so the throwing setter does not propagate: the config still loads and the appender is still attached. Verified, rather than assumed:

scenario	before this PR	after
app.config with <Dsn value="…" />	log4net:ERROR … Cannot find Property [Dsn] to set object on [Sentry.Log4Net.SentryAppender], config loads, appender attached	log4net:ERROR Failed to set parameter [Dsn] … NotSupportedException: <migration message>, config loads, appender attached
appender.Dsn = "…" in code	CS1061: SentryAppender does not contain a definition for 'Dsn'	CS0619 carrying the migration message
app.config without <Dsn>	works	works, unchanged

So the gain is a better message on the XML path and a compile error carrying migration guidance on the code path — not the hard startup failure Serilog and NLog get. Both messages go to Console.Error via log4net's internal logging unless quiet mode is on.

If a hard failure matters more than keeping the appender working, the alternative is to record the attempt in the setter and throw from ActivateOptions. ParseAppender catches that too, but it drops the appender entirely, so nothing reaches Sentry at all — worse for anyone who initializes correctly but leaves a stale <Dsn> behind. I went with the setter; say the word if you'd rather have the louder one.

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.
• Environment gets no tombstone, following feat: NLog target no longer initializes the SDK #5585, which tombstoned only the SDK-init settings. It's a one-line addition if you'd rather it carried a message too — log4net reports the removal either way, just less usefully.
• Like NLog, log4net needs no UseLog4Net(): identity and properties are applied by the appender to the events and logs it creates.
• 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

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.60%. Comparing base (a8a1ea9) to head (ba78b29).

Files with missing lines	Patch %	Lines
src/Sentry.Log4Net/SentryAppender.cs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                           @@
##           feat/no-init-from-logging-nlog-5245    #5592      +/-   ##
=======================================================================
- Coverage                                74.64%   74.60%   -0.04%     
=======================================================================
  Files                                      514      514              
  Lines                                    18760    18732      -28     
  Branches                                  3647     3640       -7     
=======================================================================
- Hits                                     14003    13975      -28     
  Misses                                    3889     3889              
  Partials                                   868      868              

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[jamescrosswell]

This gets handled here now (just a couple of lines up):

sentry-dotnet/src/Sentry.Log4Net/SentryAppender.Structured.cs

Line 20 in 6438db5

log.SetDefaultAttributes(options, scope, Sdk);