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/cli/src/crewai_cli/command.py

@@ -20,13 +20,11 @@ class AuthenticationRequiredError(SystemExit):
 class BaseCommand:
     def __init__(self) -> None:
         self._telemetry = Telemetry()
-        self._telemetry.set_tracer()
 
 
 class PlusAPIMixin:
     def __init__(self, telemetry: Telemetry) -> None:
         try:
-            telemetry.set_tracer()
             self.plus_api_client = PlusAPI(api_key=get_auth_token())
         except Exception:
             telemetry.deploy_signup_error_span()