Add WriteTo.FallbackChain and WriteTo.Fallible support in configuration

[ArieGato]

Closes #473.

Summary

• Add FallbackChain and Fallible surrogates to SurrogateConfigurationMethods so the two LoggerSinkConfiguration instance methods introduced in Serilog 4.1 can be used from appsettings.json / IConfiguration.
• Extend the Sample project with an AlwaysFailingSink plus a SampleFailureListener so both sections in the sample appsettings.json exercise the fallback path and failure-listener path at runtime.
• Add three configuration-level tests covering: primary → fallback routing, subsequent fallbacks via params array, and Fallible routing failures to a listener.

Implementation notes

Both methods are instance methods on LoggerSinkConfiguration rather than extension methods, so the extension-method scanner in ConfigurationReader.FindConfigurationExtensionMethods doesn't discover them — same as the existing Sink / Logger surrogates.

FallbackChain's third parameter is params Action<LoggerSinkConfiguration>[]. Because params does not set ParameterInfo.HasDefaultValue = true, it isn't treated as omittable by HasImplicitValueWhenNotSpecified, so the surrogate uses = null explicitly. Same underlying mechanical issue as #441; that broader fix would let the surrogate drop the ?? [] workaround later.

LoggerAuditSinkConfiguration does not expose these methods in Serilog (by design — audit pipelines are meant to fail loudly), so no corresponding AuditTo surrogates are added.

Test plan

• dotnet build passes on net462 / netstandard2.0 / net8.0 / net9.0 / net10.0
• Full test suite: 325 tests on net8/net10, 326 on net9, 0 failures
• Coverage on the two new surrogate methods: 100% line, 100% branch (measured with coverlet.collector locally)
• Sample runs end-to-end and the fallback/failure-listener paths are visibly exercised ([fallback ...] lines in console and [SampleFailureListener] lines in SelfLog)

[nblumhardt]

Looks good, thanks @ArieGato.