Propagate authenticated user name from HTTP to Marten LastModifiedBy

[jeremydmiller]

Summary

• Adds EnableRelayOfUserName option to WolverineOptions that, when enabled, automatically extracts HttpContext.User.Identity.Name and propagates it through the messaging infrastructure
• Sets IMessageContext.UserName from the authenticated ClaimsPrincipal, carries it on outgoing Envelope.UserName, and applies it to Marten's IDocumentSession.LastModifiedBy
• Adds OpenTelemetry enduser.id tag on the current activity

Changes

• Core: Added UserName property to Envelope, IMessageContext, MessageBus, MessageContext, TestMessageContext, and EnvelopeConstants
• Serialization: EnvelopeSerializer reads/writes UserName for binary round-trip
• Relay: MessageBus.TrackEnvelopeCorrelation() propagates UserName to outgoing envelopes when enabled
• HTTP Middleware: New UserNameMiddleware + UserNamePolicy (follows RequestIdMiddleware pattern) registered in WolverineHttpOptions
• Marten: OutboxedSessionFactory.configureSession() sets session.LastModifiedBy from UserName
• Docs: Added "Tracking User Name" section to HTTP security documentation

Test plan

• Unit: EnvelopeSerializer round-trip for UserName (set and null)
• Unit: TrackEnvelopeCorrelation relays UserName when enabled, skips when disabled, doesn't override existing
• Unit: ReadEnvelope reads UserName from incoming envelope
• Integration: HTTP endpoint returns UserName from authenticated ClaimsPrincipal
• Integration: Returns "NONE" when not authenticated

Closes #2203

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