Outbox telemetry: rename ActivitySource + auto-register when tracing is enabled

[Vulthil]

Summary

Makes the outbox relay's tracing work out of the box. Follows #232 (merged).

• Rename the relay ActivitySource from "Vulthil.SharedKernel.Infrastructure" to "Vulthil.SharedKernel.Outbox" (Telemetry.ActivitySourceName) so it matches the package that emits it.
• Auto-register the source with OpenTelemetry from AddOutboxEngine (← EnableOutboxProcessing), gated on OutboxProcessingOptions.EnableTracing (default on) — so OutboxPublishing spans reach the app's configured tracer with no manual AddSource/wildcard. When EnableTracing is off, registration is skipped entirely (and the processor emits no spans either).
• AddVulthilOutboxInstrumentation() TracerProviderBuilder helper (mirrors the RabbitMq package's AddVulthilMessagingInstrumentation()) for manual tracer-provider setups; the engine calls it internally.
• OutboxProcessingOptions.EnableTracing is now settable (init → set) so the gate can read it from the configure callback.
• Adds OpenTelemetry.Api + OpenTelemetry.Extensions.Hosting to the (previously OTel-free) engine package. PublicAPI + docs updated.

Verification

Full Release build clean; WebApi.Tests 8/8 (confirms the engine's AddOpenTelemetry() merges with the app/ServiceDefaults' own at runtime).

[claude]

Code review

One issue found while checking for bugs and CLAUDE.md compliance.

XML doc not updated for the init → set change on EnableTracing

In OutboxProcessingOptions.cs the EnableTracing accessor changed from init to set, so it is now a read/write property — but the XML doc summary still reads "Gets a value indicating whether…".

Per the project rule — "When modifying a public member, make sure to update the XML Documentation" — and this file's own convention (every init property is documented with "Gets"), the summary should be "Gets or sets":

    /// Gets or sets a value indicating whether Trace Identifiers should be included when publishing outbox messages.

(Posted as a summary comment — inline review comments are not available in this environment.)