chore: refresh the semconv fixture from the Python SDK

Picks up SERVICE_INSTANCE_ID = "service.instance.id", which Python gained
when service.instance.id shipped here in #24.

Purely a stale mirror: src/semconv.ts already exports that constant with
that value, so the parity test never had anything to catch — the test walks
the FIXTURE's keys, and a key the fixture lacks is simply never compared.
Only the weekly regenerate-and-diff check saw it, which is what opened #25.

Closes #25

Author: PabloPardoGarcia

tests/fixtures/semconv.json

@@ -1,5 +1,6 @@
 {
   "TRACER_NAME": "glassflow",
+  "SERVICE_INSTANCE_ID": "service.instance.id",
   "OPENINFERENCE_SPAN_KIND": "openinference.span.kind",
   "INPUT_VALUE": "input.value",
   "OUTPUT_VALUE": "output.value",