test(ttp): E.2.14a follow-up — force DECNET_DEVELOPER_TRACING=true, skip when Jaeger unreachable

Session-scoped autouse fixture in tests/ttp/conftest.py sets
DECNET_DEVELOPER_TRACING=true and forces decnet.telemetry._ENABLED
so the no-op tracer doesn't silently swallow emitted spans. The
span_exporter fixture also monkeypatches decnet.telemetry.get_tracer
so production code under test lands spans in the in-memory
exporter. Tracing tests skip when DECNET_OTEL_ENDPOINT (default
localhost:4317) isn't reachable so the dev loop stays green
without lying about coverage.

tests/ttp/conftest.py

@@ -0,0 +1,41 @@
+"""Shared TTP test fixtures.
+
+Forces OTEL tracing ON for all tests in ``tests/ttp/``. Without this
+shim :mod:`decnet.telemetry` evaluates ``_ENABLED`` once at import
+time from the ``DECNET_DEVELOPER_TRACING`` env var; if the suite is
+invoked without that var set, every call to ``get_tracer()`` returns
+the no-op stand-in and span-emission tests (E.2.14a, future E.3
+impl tests) silently capture nothing.
+
+Two complementary moves:
+
+1. Set ``DECNET_DEVELOPER_TRACING=true`` in :func:`os.environ` before
+   any tests run. Catches the case where a fresh import of
+   ``decnet.telemetry`` happens after collection.
+2. Mutate the already-imported ``decnet.telemetry._ENABLED`` flag to
+   ``True``. Catches the case where the module was already imported
+   (e.g. by another test or fixture) before this conftest ran —
+   reload-and-pray races are nasty enough to hardcode the override.
+
+Both are session-scoped and autouse — the cost of an active OTEL
+provider in unrelated tests is negligible (the SDK no-ops when no
+processor is attached).
+"""
+from __future__ import annotations
+
+import os
+
+import pytest
+
+
+@pytest.fixture(scope="session", autouse=True)
+def _enable_decnet_tracing() -> None:
+    """Force ``DECNET_DEVELOPER_TRACING=true`` for the test session.
+
+    Set env var first (covers late imports), then poke
+    ``decnet.telemetry._ENABLED`` directly (covers already-imported
+    case). Either alone is racy; both together is robust.
+    """
+    os.environ["DECNET_DEVELOPER_TRACING"] = "true"
+    import decnet.telemetry as _t  # noqa: PLC0415 — fixture-time import
+    _t._ENABLED = True