fix(common): coerce null/empty/whitespace observation Result to UNAVAILABLE

[ottobolyos]

Summary

MTConnectAgent.AddObservation accepted observations whose Result value was null, the empty string "", or whitespace-only and forwarded that value verbatim onto every wire transport (HTTP /current, /sample, MQTT JSON-cppagent, SHDR). MTConnect Part 1 §Observation Information Model — Representation — Observation Values mandates "UNAVAILABLE" as the sole valid representation of a missing or undetermined observation value:

"If an Agent cannot determine a Valid Data Value for a DataItem, the value returned for the Result for the Data Entity MUST be reported as UNAVAILABLE."

The empty string is, by definition, not a Valid Data Value (it parses as no value at all under every typed DataItem schema). This PR adds an unconditional coerce step in the canonical AddObservation(string deviceKey, IObservationInput, …) overload — null / empty / whitespace Result is rewritten to Observation.Unavailable before validation, so:

• Ignore / Warning / Remove: publishes UNAVAILABLE rather than the empty value, satisfying the spec mandate on the wire.
• Strict: coerces to UNAVAILABLE, the observation lands rather than being silently dropped.

The coerce skips CONDITION observations (their Level enum cannot carry the empty pathology). DATA_SET / TABLE / TIME_SERIES Representation pre-fill of Count / SampleCount runs after the coerce — semantics for those representations are unchanged.

No opt-out flag is exposed: the spec mandate is MUST, so a way to opt out of compliance would itself be non-conformant.

Behaviour change

Input	Pre-fix wire payload	Post-fix wire payload
Result = null	"" (empty)	"UNAVAILABLE"
Result = ""	""	"UNAVAILABLE"
Result = " " (whitespace)	" "	"UNAVAILABLE"
Result = "AVAILABLE" (concrete)	"AVAILABLE"	"AVAILABLE" (unchanged)
Result = "" under Strict	observation silently dropped	lands with "UNAVAILABLE"

Tests

Unit (NUnit, tests/MTConnect.NET-Common-Tests/Agents/AddObservationEmptyResultCoerceTests.cs, 10 cases):

• empty-string Result under every non-Strict InputValidationLevel
• the null / "" / whitespace family under Warning
• empty Result under Strict (lands, not silently dropped)
• concrete-value preservation (negative — the coerce does not substitute for a Valid Data Value)

Wire-level E2E (xUnit, [Trait("Category", "E2E")]):

tests/MTConnect.NET-Integration-Tests/Workflows/AddObservationEmptyResultUnavailableWorkflowTests.cs — HTTP /current:

• null / empty / spaces / tab / newline / CRLF Result → AVAILABILITY element body is UNAVAILABLE (6-case [Theory])
• concrete value → AVAILABILITY element body is verbatim
• concrete → empty sequence → AVAILABILITY element reflects the latest (UNAVAILABLE)
• empty Result under Strict validation → AVAILABILITY element body is UNAVAILABLE (lands, not silently dropped; spins its own agent + server harness)

tests/MTConnect.NET-Integration-Tests/Workflows/AddObservationEmptyResultUnavailableSampleStreamWorkflowTests.cs — HTTP /sample stream:

• null / empty / spaces / tab / newline / CRLF Result → latest AVAILABILITY sample is UNAVAILABLE (6-case [Theory])
• concrete value → latest AVAILABILITY sample is verbatim
• concrete → empty sequence → two distinct samples in stream order (concrete first, then UNAVAILABLE); proves the coerce does not silently drop the second observation

All wire surfaces fed by the agent's buffer read the same coerced payload — every transport (HTTP, MQTT relay, SHDR adapter output) inherits the fix because the coerce sits above the buffer.

Files touched

• libraries/MTConnect.NET-Common/Agents/MTConnectAgent.cs — invoke the coerce in AddObservation; add IsEmptyResult + CoerceEmptyResultToUnavailable helpers.
• tests/MTConnect.NET-Common-Tests/Agents/AddObservationEmptyResultCoerceTests.cs — new unit fixture.
• tests/MTConnect.NET-Integration-Tests/Workflows/AddObservationEmptyResultUnavailableWorkflowTests.cs — new wire-level E2E fixture against HTTP /current.
• tests/MTConnect.NET-Integration-Tests/Workflows/AddObservationEmptyResultUnavailableSampleStreamWorkflowTests.cs — new wire-level E2E fixture against HTTP /sample stream.
• .github/workflows/dotnet.yml — drop matrix expressions from the two matrix jobs' name: fields so skipped-state status checks render cleanly (build-and-test / route-check-e2e instead of build-and-test-${{ matrix.os }}).

[ottobolyos]

@PatrickRitchie, this PR fixes a Part 1 spec violation that touches every wire output: MTConnectAgent.AddObservation was forwarding null / empty / whitespace Result values verbatim, where Part 1 §Observation Information Model — Representation — Observation Values mandates UNAVAILABLE as the sole valid representation of a missing value. The fix is small and self-contained—4 files across 4 commits, with 10 unit + 17 wire-level E2E cases pinning the contract—but the bug class is broad enough (every HTTP envelope, MQTT relay, and SHDR adapter output benefits from the coerce at the buffer entry) that a review at your convenience would be appreciated.

[ottobolyos]

@PatrickRitchie — heads-up: while validating PR #189's dime-connector build I caught an unrelated multi-TFM Release-pack regression introduced by an earlier warnings sweep. The minimal fix lands in PR #194 (just opened as draft); the broader warnings cleanup plus a Release-pack CI gate ships in PR #195.