[Types] Complete ExtensionObject decoding and add an atomic file replace capability - #4128

…bility

ExtensionObject.TryGetValue took an IServiceMessageContext but never used it:
the body contained a "TODO: Decode if possible" and returned false whenever the
payload was still raw. It now decodes binary, XML and JSON bodies through the
codecs, resolving the concrete type from the context's IEncodeableFactory, so no
parameterless constructor constraint is needed. Behaviour without a context is
unchanged.

Variant's structure helpers now delegate to that instead of carrying a second
implementation of the same logic.

IAtomicFileReplace is a new optional capability for IFileSystem implementations
that can publish a fully written file under its final name in one indivisible
step. It is deliberately a separate interface rather than a member on
IFileSystem, because that interface is implemented outside this repository and
the library targets frameworks without default interface members. LocalFileSystem
implements it with File.Replace or File.Move, and VirtualFileSystem by re-keying
its in-memory entry. Durable writers can then stage content under a temporary
name and publish it, so an interrupted write can never leave a partially written
file visible at the destination.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 9e6a5abf-3299-4cd1-9855-010fedbf0ad8

Replace the separate optional IAtomicFileReplace capability with a Replace member on IFileSystem at the maintainer's request, and implement the member on all in-repo file system implementations.

Update VirtualFileSystem.Replace to use a TryGetValue/TryUpdate/TryAdd retry loop so the dictionary update is conditional and exactly the replaced entry is disposed after a successful update.

Use File.Move with overwrite on modern TFMs for LocalFileSystem.Replace so overwrites work on platforms where File.Replace is unavailable, while keeping the File.Replace fallback for net48.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 9e6a5abf-3299-4cd1-9855-010fedbf0ad8

…ore-types

Explain why the ExtensionObject TypeId path checks known datatype ids before falling back to encoding lookup.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 9e6a5abf-3299-4cd1-9855-010fedbf0ad8

PubSubApplication.StartAsync used to log connection EnableAsync failures and continue, leaving the application marked started even though the failed connection had no transport. Discovery and Action callers then saw misleading invalid-operation errors instead of the underlying transport failure. Re-throw enable failures after stopping the partially-started application so environment failures surface at startup.

A channel-manager lease swapped from a terminal entry to an already-open fresh entry missed the fresh entry's current state transition because the lease was not attached while OpenInitialAsync moved the entry to Ready. ReattachParticipant now replays the fresh entry's current observable state to the lease, so ManagedSession.ChannelStateChanged consumers see Ready after recovery.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 9e6a5abf-3299-4cd1-9855-010fedbf0ad8