Fix SDK OpenTelemetry opt-in behavior

[rbarbadillo]

Summary

This fixes SDK OpenTelemetry opt-in behavior when an application already has a global OpenTelemetry provider configured.

The SDK now emits Mistral SDK GenAI spans only when Mistral telemetry is explicitly enabled, for example with MISTRAL_SDK_TELEMETRY=global or a per-client telemetry provider. Application-owned HTTP instrumentation continues to behave independently.

Behavior

• No MISTRAL_SDK_TELEMETRY: no Mistral SDK GenAI span is emitted.
• MISTRAL_SDK_TELEMETRY=false: no Mistral SDK GenAI span is emitted.
• MISTRAL_SDK_TELEMETRY=global: SDK GenAI spans use the application’s global OpenTelemetry provider.
• Custom/per-client tracer providers continue to work as before.

Testing

PYTHONPATH=src:packages/azure/src:packages/gcp/src .venv/bin/python -m ruff check \
  src/mistralai/client/_hooks/tracing.py \
  src/mistralai/extra/observability/telemetry.py \
  src/mistralai/extra/tests/test_otel_tracing.py

PYTHONPATH=src:packages/azure/src:packages/gcp/src .venv/bin/python -m pytest \
  src/mistralai/extra/tests/test_telemetry.py \
  src/mistralai/extra/tests/test_otel_tracing.py