feat: NLog target no longer initializes the SDK

[jamescrosswell]

The NLog portion of #5245, stacked on #5573 (Serilog) and following the same design. The Sentry target for NLog now only configures the target; Sentry has to be initialized separately via SentrySdk.Init, UseSentry, etc.

Part of #5245

Breaking changes

• SentryNLogOptions no longer derives from SentryOptions. It carries only target settings.
• Removed from SentryNLogOptions and SentryTarget: InitializeSdk, Dsn/DsnLayout, Release/ReleaseLayout, Environment/EnvironmentLayout, ShutdownTimeoutSeconds, FlushTimeout/FlushTimeoutSeconds. Events now take release and environment from the options used to initialize Sentry, rather than per-target overrides.
• When NLog flushes the target, Sentry is flushed using SentryOptions.FlushTimeout from the options used to initialize Sentry. That defaults to 2 seconds; the NLog target previously defaulted to 15. Set FlushTimeout in SentrySdk.Init to keep the old wait.
• In NLog.config, the dsn, release, environment, initializeSdk, shutdownTimeoutSeconds and flushTimeoutSeconds target attributes are gone, as is setting arbitrary SentryOptions properties through the <options> element (e.g. <options attachStacktrace="true" />). With throwConfigExceptions="true" these now fail config loading.
• The three AddSentry overloads collapse into one: AddSentry(Action<SentryNLogOptions>? optionsConfig = null, string targetName = "sentry").
• SDK diagnostics are no longer written to NLog's InternalLogger. Previously, enabling NLog internal logging at any level made the target set the SDK's DiagnosticLogger to write there and turn on Debug. To see SDK diagnostics, set Debug (and optionally DiagnosticLogger) on the options used to initialize Sentry.

Before:

LogManager.Configuration = new LoggingConfiguration()
    .AddSentry("https://key@sentry.io/1", o => o.MinimumEventLevel = LogLevel.Error);

After:

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

LogManager.Configuration = new LoggingConfiguration()
    .AddSentry(o => o.MinimumEventLevel = LogLevel.Error);

Notes for review

• targetName moved to the last parameter on purpose. Keeping a (string targetName, Action<SentryNLogOptions>) overload would let existing AddSentry(dsn, o => …) calls keep compiling, with the DSN silently used as the target name and Sentry never initialized. With targetName last, every old DSN-taking call fails to compile instead.
• Why the flush timeout moved to the SDK options. NLog flushes its targets on LogManager.Flush(), on LogManager.Shutdown() (which NLog also calls itself on process exit, since AutoShutdown is on by default), when the configuration is replaced (including autoReload), and from wrappers such as AutoFlushTargetWrapper. NLog doesn't pass its own timeout down to targets, so the target has to pick one, and the SDK-wide flush it triggers covers events from every integration, not only NLog. Previously the target owned the SDK, so this flush on shutdown was how an NLog-only app got its last events sent. Now the app initializes and disposes Sentry itself, and disposing the handle from SentrySdk.Init flushes on its own. The NLog-triggered flush is still useful for explicit flushes, config reloads and auto-flush wrappers, but it's an SDK operation, so it uses the SDK's setting instead of a target-level duplicate.
• Why NLogDiagnosticLogger is deleted. Routing SDK diagnostics to InternalLogger was SDK configuration applied at target initialization. The target's options are no longer SentryOptions, and applying it to the live hub options instead would mean the target silently reconfiguring (and switching on Debug for) an SDK the user initialized. With no remaining callers, the internal class was dead code.
• Unlike Serilog, NLog needs no UseNLog(): tags, user and properties are all applied by the target to the events it creates, and the SDK name is stamped per event.
• The IntegrationTests.Simple snapshot changes are only a stack-frame line/column shift from restructuring the test. Net4_8 got the same edit by hand, and ApiApprovalTests.Run.Net4_8 is a copy of the regenerated DotNet10_0 one, which it matched byte-for-byte beforehand. Neither can regenerate on macOS.
• The integration tests previously never disposed the SDK the target initialized; they now dispose it via using before verifying.

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[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.58%. Comparing base (c72c26e) to head (489d7ce).

Files with missing lines	Patch %	Lines
src/Sentry.NLog/SentryTarget.cs	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@                        Coverage Diff                         @@
##           feat/no-init-from-logging-5245    #5585      +/-   ##
==================================================================
- Coverage                           74.67%   74.58%   -0.09%     
==================================================================
  Files                                 515      514       -1     
  Lines                               18834    18745      -89     
  Branches                             3667     3646      -21     
==================================================================
- Hits                                14064    13981      -83     
+ Misses                               3893     3891       -2     
+ Partials                              877      873       -4     

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