ref: Stop returning StreamedSpan from get_start_span_function() (#6560)

The new `traces.start_span()` API automatically promotes spans to segment spans if there is no active segment.
Since `get_start_span_function()` is never invoked when span streaming is enabled, the function does not need to support the streaming trace lifecycle.

Author: alexander-alderman-webb

sentry_sdk/ai/utils.py

@@ -13,7 +13,6 @@
 
 import sentry_sdk
 from sentry_sdk.traces import StreamedSpan
-from sentry_sdk.tracing_utils import has_span_streaming_enabled
 from sentry_sdk.utils import logger
 
 MAX_GEN_AI_MESSAGE_BYTES = 20_000  # 20KB
@@ -538,13 +537,7 @@ def normalize_message_roles(messages: "list[dict[str, Any]]") -> "list[dict[str,
 
 
 def get_start_span_function() -> "Callable[..., Any]":
-    if has_span_streaming_enabled(sentry_sdk.get_client().options):
-        return sentry_sdk.traces.start_span
-
     current_span = sentry_sdk.get_current_span()
-    if isinstance(current_span, StreamedSpan):
-        # mypy
-        return sentry_sdk.traces.start_span
 
     transaction_exists = (
         current_span is not None and current_span.containing_transaction is not None