feat(http): expose probe device model via Devices accessor

[ottobolyos]

Summary

Closes #176. Adds a public Devices read-only accessor on MTConnectHttpClient that returns a snapshot of the post-probe device tree, and fixes DeviceReceived to fire for every parsed device. The previous implementation built an empty outputDevices list, populated _devices only, then iterated the empty list, so the event was raised zero times across the lifetime of every client. Together the two changes give consumers two clean paths to the post-processed device model — event-driven via DeviceReceived, snapshot-on-demand via Devices.

Beyond the headline fix, the branch lands the review-finding follow-ups noted by the maintainer on this PR:

• The probe-time cache is now cleared at the start of every probe, so devices the agent has stopped advertising are evicted before the new probe loads.
• DeviceReceived is fanned out per-subscriber through an invocation-list walk; a throwing handler can no longer abort the populate loop or starve later subscribers. Each fault is forwarded through InternalError, and a faulting InternalError handler is itself isolated so the fan-out survives.
• The Devices snapshot is wrapped in ReadOnlyDictionary so a consumer cannot mutate the cache through a downcast.
• The XML docs for both surfaces are tightened and the corresponding narrative pages (docs/configure/consumer.md, docs/examples/client-http.md) are extended to describe the new accessor and event.

Files changed

• libraries/MTConnect.NET-HTTP/Clients/MTConnectHttpClient.cs — add public IReadOnlyDictionary<string, IDevice> Devices returning a lock-guarded snapshot; restructure ProcessProbeDocument to clear the cache and raise DeviceReceived inside the same loop that populates _devices, dropping the dead outputDevices list and the never-fired second loop; add RaiseDeviceReceived to walk the invocation list with per-delegate try/catch and route faults through InternalError.
• tests/MTConnect.NET-HTTP-Tests/Clients/HttpClientDeviceModel.cs — new NUnit fixture pinning the Devices accessor and the DeviceReceived event end to end against the embedded AgentRunner HTTP server. Ten tests cover the positive surface (empty-before-probe, populated-after-probe, snapshot-not-live-view, fires-once-per-device, carries-wired-back-pointers, fires-on-subsequent-probes, evicts-stale-entries-on-reprobe) and the negative surface (throwing handler does not break cache population; throwing handler does not starve later subscribers; throwing InternalError does not break the DeviceReceived fan-out).
• docs/examples/client-http.md — adds a "Subscribing to the wired device model" subsection showing a DeviceReceived handler walking the wired DataItems and a Devices snapshot read after the first probe completes.
• docs/configure/consumer.md — names Devices and DeviceReceived alongside the existing CurrentReceived guidance, with a note on the InternalError fault routing.

Test plan

• dotnet test tests/MTConnect.NET-HTTP-Tests --filter "FullyQualifiedName~HttpClientDeviceModel" — all ten tests pass.
• dotnet build MTConnect.NET.sln — clean build on the rebased head.
• dotnet test MTConnect.NET.sln --filter "Category!=E2E&Category!=RequiresDocker&Category!=XsdLoadStrict" — full HTTP-client suite continues to pass alongside the rest of the matrix.