fix(tracing): Skip child span creation in streaming path when no current span (web frameworks)

[sentrivana]

Summary

• When span streaming is enabled and there is no current span, web framework integrations should not create new root segments for child-span operations
• Adds a sentry_sdk.traces.get_current_span() is None guard before creating spans in the streaming path
• Affected integrations: starlette, starlite, litestar, graphene, strawberry, aiohttp

Test plan

• Existing tests pass
• Verify that in streaming mode, no orphan root segments are created for web framework sub-operations when there's no active span

🤖 Generated with Claude Code

[sentry-warden]

Early return skips trace propagation headers when no current span in streaming mode

The early return when there is no current span skips the should_propagate_trace/iter_trace_propagation_headers block, so outgoing aiohttp requests lose distributed-trace headers that would otherwise be derived from the scope's propagation context.

Evidence

• scope.iter_trace_propagation_headers (scope.py:680) falls through to get_active_propagation_context().iter_headers() and still yields headers even when span is None.
• The new guard at lines 356-358 sets trace_config_ctx.span = None and returns before reaching the should_propagate_trace(...) header-injection block later in on_request_start.
• In the non-streaming path a span is always created, so propagation headers are always added; the streaming no-span path now bypasses propagation entirely.

_{Identified by Warden code-review · 4TJ-8ZY}