fix: stop anonymous telemetry from globalizing the TracerProvider

`Telemetry.set_tracer()` installed crewAI's anonymous SDK
`TracerProvider` into OpenTelemetry's process-global slot, so the first
`Crew` constructed in a test or host application replaced the default
`ProxyTracerProvider` and exfiltrated every host span emitted via
`trace.get_tracer(...)` to crewAI's OTLP endpoint. Keep the provider
local to the `Telemetry` instance and route every anonymous span
through `self.provider.get_tracer("crewai.telemetry")` so the global
slot stays untouched. Mirrors the fix in `crewai_core.telemetry`,
drops the now-dead `set_tracer()` calls in `event_listener.py` and
`crewai_cli.command`, and adds regression coverage that asserts the
provider stays a `ProxyTracerProvider` after constructing a `Crew`.

lib/crewai/tests/telemetry/test_otel_noop.py

@@ -51,6 +51,21 @@ def test_operation_yields_non_recording_span_when_no_provider() -> None:
         assert isinstance(span, NonRecordingSpan)
 
 
+def test_constructing_crew_does_not_globalize_anonymous_telemetry_provider() -> None:
+    agent = Agent(
+        role="tester",
+        goal="goal",
+        backstory="backstory",
+        llm=_FakeLLM(),
+        allow_delegation=False,
+    )
+    Crew(
+        agents=[agent],
+        tasks=[Task(description="d", expected_output="o", agent=agent)],
+    )
+    assert isinstance(trace.get_tracer_provider(), ProxyTracerProvider)
+
+
 def test_kickoff_runs_cleanly_without_provider() -> None:
     agent = Agent(
         role="tester",