Return clear error for non-finite (NaN/Inf) floats in KServe REST output

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Summary

The REST output serializers silently produced malformed JSON when a model output contained non-finite floats (NaN/±Inf). Floats were written via rapidjson Writer::Double() while ignoring its return value — rapidjson writes nothing for non-finite values (they are invalid per the JSON spec and the default writer has no kWriteNanAndInfFlag), so the affected array element was dropped, corrupting the response. There was no isfinite guard anywhere in the REST output path.

This now returns a clear JSON_SERIALIZATION_ERROR with a descriptive message instead of emitting broken JSON, on both the KServe v2 and the TFS REST paths.

Change

• Added an isJsonRepresentable helper (overloaded: integral types always true; float/double use std::isfinite).
• KServe v2 path: guarded both write paths of the PARSE_OUTPUT_DATA macro (val-field and raw-output) to return JSON_SERIALIZATION_ERROR with Output "<name>" contains a non-finite (NaN/Inf) value that cannot be serialized to JSON.
• TFS path: applied the same finiteness check to makeJsonFromPredictResponse (PredictResponse), validating float/double outputs before MakeJsonFromTensors and returning the same error.
• Integer outputs are unaffected (helper resolves to the always-true template; negligible cost).
• Added unit tests for Inf, NaN, and the unaffected-integer case on both paths.

Validation

Built ovms_test (Windows, py-off) and ran the serialization suite — all pass:

• Full *MakeJsonFromPredictResponse* suite: 85/85 (KFS + TFS, raw + val-field) — no regression.
• New non-finite tests: 3 KFS + 4 TFS (Inf/NaN float, double, int-unaffected) all pass.

Context

Found while triaging #2337 (a model emitting -Inf/NaN on GPU). That issue's root cause is upstream/model-side and appears resolved on current OpenVINO; this PR is the independent OVMS-side hardening so that any model producing non-finite values fails cleanly over REST instead of returning corrupt JSON.

Relates to #2337.

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Update: extended the same non-finite (NaN/Inf) guard to the TFS REST serialization path as well (it was originally flagged as a possible follow-up). Both REST paths are now covered, and the description/tests have been updated accordingly — full *MakeJsonFromPredictResponse* suite now 85/85 (KFS + TFS).