fix(mtconnect): make a /sample stream end loud, and correct the gap contract (Task 7 review)

Code review of c46540ae approved the framing algorithm but moved the risk
onto the contract around it.

C1 (critical). SampleAsync returned normally on three different events:
EOF, the closing --boundary--, and a response that was not multipart at
all (where it yielded one parsed snapshot and finished). The seam
documents the stream as yielding until ct is cancelled, so a Task 11 pump
written to that contract would silently drop its subscription or hot-loop
reconnecting; the non-multipart leg reported a configuration error as a
healthy finished stream, which is the #485 quiet-successful-termination
shape aimed at the very next task. Every non-cancellation end now throws:
MTConnectStreamEndedException (ConnectionClosed / ClosingBoundary -
transient) or MTConnectStreamNotSupportedException (configuration -
reconnecting reproduces it forever), sharing one catchable base. The
non-multipart body is still parsed first so an Agent's own MTConnectError
text wins, but the document is NOT yielded.

I1. IsSequenceGap moved to IMTConnectAgentClient (static) and its first
parameter is now expectedFrom. The old name and doc were wrong for every
chunk after the first - the correct comparand is the PREVIOUS chunk's
NextSequence, and comparing against the opening "from" argument reports a
gap on every chunk once the ring buffer rolls, i.e. an endless /current
re-baseline storm. Both doc blocks also now record that cppagent answers
from < firstSequence with an OUT_OF_RANGE MTConnectError under HTTP 200,
which surfaces as InvalidDataException and NOT as a gap - so Task 11 must
treat that parse failure as a re-baseline trigger too.

I2. Every multipart test served the whole body from one buffer, so every
framing test completed in a single ReadAsync - the split-boundary path,
the split-header path and the multi-fill loop were correct by inspection
only. A ChunkedStream double (N bytes per read, N = 1/3/7) now drives the
fixtures through a split transport, plus a >16 KB document that forces a
buffer resize. Falsifiability: removing either scan overlap, or
collapsing the fill loop, fails ONLY the new chunked tests.

I3. Disposal mid-enumeration surfaces as OperationCanceledException via
an internal dispose-linked token, not a raw ObjectDisposedException that
Task 9's re-init would inflict on an enumerating pump.

I5. HeartbeatMs, SampleIntervalMs and SampleCount join RequestTimeoutMs
in construction-time positive-value validation. All three reach the query
string and an Agent answers heartbeat=0 with an HTTP-200 MTConnectError
that names no config key.

I4/M2. The no-Content-length framing fallback logs a one-shot warning
(the client takes an optional ILogger); a multipart response with no
boundary parameter fails fast instead of degrading into a watchdog
timeout.

M5/M6. Shared element/attribute reading rules extracted to MTConnectXml;
the probe parser documents why it alone does not trim BOM/whitespace. The
literal U+FEFF in source became a '' escape.

Also, from Task 8: MTConnectObservation gained IsStructured (default
false), set from element.HasElements. A DATA_SET/TABLE observation's
<Entry> children concatenate through element.Value to nonsense ("12" for
two entries) that the index would publish as Good, and only the parser
can tell that apart from a legitimate space-bearing Message. False for
CONDITION observations - their value comes from the element name, so
child content cannot corrupt it.

275/275 green in this suite's own files.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectDtos.cs

@@ -136,10 +136,33 @@ public sealed record MTConnectStreamsResult(
 ///     The observation's Agent-reported timestamp, normalized to UTC (<see cref="DateTime.Kind"/>
 ///     is always <see cref="DateTimeKind.Utc"/>). Becomes the OPC UA variable's SourceTimestamp.
 /// </param>
+/// <param name="IsStructured">
+///     <c>true</c> when the Agent carried this observation's real content in <b>child elements</b>
+///     rather than as text — an MTConnect 2.0 <c>DATA_SET</c> / <c>TABLE</c> observation, whose
+///     content is a list of <c>&lt;Entry key="…"&gt;</c> elements.
+///     <para>
+///         <b>Why the flag exists, and why only the parser can set it.</b> <see cref="Value"/> is
+///         the element's concatenated descendant text, so a two-entry data set reads as the single
+///         token <c>"12"</c> — keys discarded, values run together. The observation index cannot
+///         detect that after the fact: neither this record nor the tag definition carries the
+///         DataItem's <c>representation</c>, and no value-shape heuristic can work, because
+///         concatenated entries are indistinguishable from a legitimate space-bearing EVENT such as
+///         a <c>Message</c> reading <c>"Coolant level low"</c>. The one reliable discriminator —
+///         that the observation element had element children — exists only while the XML is still
+///         XML.
+///     </para>
+///     <para>
+///         Deliberately a <b>neutral fact about the wire shape</b>, not a status: mapping it to a
+///         quality code is the observation index's job (it codes these
+///         <c>BadNotSupported</c> rather than publishing concatenated noise as Good). Defaults to
+///         <c>false</c>, the shape of every ordinary scalar observation.
+///     </para>
+/// </param>
 public sealed record MTConnectObservation(
     string DataItemId,
     string Value,
-    DateTime TimestampUtc);
+    DateTime TimestampUtc,
+    bool IsStructured = false);
 
 /// <summary>
 ///     Recursive helpers over the <see cref="IMTConnectComponentContainer"/> device/component tree.