Python: fix: establish correct span parenting during streaming agent invocation

[JasonOA888]

Problem

Closes #5089

When stream=True, the invoke_agent span is not activated as the current OTel context. Child spans (chat, execute_tool) are created without a parent reference and appear as siblings instead of children.

Expected:                      Actual (broken):
invoke_agent (WeatherAgent)    invoke_agent (WeatherAgent)
  ├── chat (gpt-5.4)           chat (gpt-5.4)              ← sibling
  ├── execute_tool             execute_tool                ← sibling
  └── chat (gpt-5.4)           chat (gpt-5.4)              ← sibling

Root Cause

The streaming path in _trace_agent_invocation uses bare get_tracer().start_span() without context activation. The original code deliberately avoided trace.use_span() because streaming spans are closed in cleanup hooks that run in a different async context, causing "Failed to detach context" errors.

Fix

Introduce _STREAMING_AGENT_INVOKE_CONTEXT — a contextvars.ContextVar that holds the OTel context with the agent-invoke span set as current.

Three changes:

1. _trace_agent_invocation (streaming path): After creating the invoke_agent span, store trace.set_span_in_context(span) in the ContextVar. Reset in _close_span.

2. _trace_chat_response (streaming path): Read the ContextVar and pass as context= to start_span(), so chat spans become children of invoke_agent.

3. get_function_span: Read the ContextVar and pass as context= to start_as_current_span(), so execute_tool spans become children of invoke_agent.

This avoids use_span()/attach() entirely — no detach errors — while propagating parent context via Python contextvars.

Testing

• All 14 existing streaming observability tests pass
• All 90+ observability tests pass

Files Changed

• python/packages/core/agent_framework/observability.py (+32 lines)