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)

[Copilot]

Pull request overview

Fixes OpenTelemetry span hierarchy in Python streaming agent runs so chat and execute_tool spans correctly appear as children of the streaming invoke_agent span (closing issue #5089).

Changes:

• Introduces a ContextVar (_STREAMING_AGENT_INVOKE_CONTEXT) to hold an OTel Context with the streaming invoke_agent span set as current.
• Uses that stored context as the explicit parent when starting streaming chat spans.
• Uses that stored context as the explicit parent when starting execute_tool spans via get_function_span().

[Copilot]

In the streaming path, _STREAMING_AGENT_INVOKE_CONTEXT is set before any potential exceptions from _capture_messages(...)/attribute serialization. If an exception is raised here, the ContextVar token is never reset and the span is never ended, which can leak the streaming parent context into later operations in the same task. Consider wrapping the post-set(...) setup in a try/except/finally that resets the ContextVar (and ends the span) on early failures before the cleanup hooks are registered.