feat: add span customizers (SDK-316)

[Andrew Kent (realark)]

AI Generated Description

Summary

Implements SDK-316 using the span customizer specification and the Java exporter reference.

• Add extensible ISpanCustomizer.OnSpanExport and ordered configuration registration through BraintrustConfig.FromEnvironment(...) / Of(...).
• Hooks transform detached OpenTelemetry.Proto.Trace.V1.Span messages, not application Activity instances.
• Preserve IDs after each hook and across the full batch, support ordered replacement/redaction/routing, and fail closed before any transport send.
• Preserve the existing exporter’s resource/scope metadata, auth, limits, and lifecycle. No customizers means no snapshot conversion overhead.

.NET representation decision

The upstream .NET OTel exporter accepts Activity, which cannot be safely cloned with identical IDs through public APIs. An optional HTTP handler decodes its OTLP batch, runs hooks, and reserializes before transmission. This adds a decode/encode pass only when configured. Vendored official opentelemetry-proto v1.5.0 schemas retain license headers and a pinned source reference; generated public types use Google.Protobuf and private build-time Grpc.Tools. Upstream retries can invoke hooks again; this is documented.

Verification

• Customizer transport suite: 10 passed, including real upstream OtlpTraceExporter serialization and no-send failures.
• Full core test project: 197 passed.
• Release build of the entire solution succeeded.
• dotnet format Braintrust.Sdk.sln --verify-no-changes --no-restore passed.
• Standalone public API smoke with a real local HTTP receiver: redaction, preserved span identity, and unchanged application Activity.