docs(changelog): record Pydantic-AI tracing handler removal

This PR removes create_pydantic_ai_tracing_handler from the public adk
surface; document it here (the previous PR's entry was narrowed to the
LangGraph handler it actually removed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: declan-scale

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### ⚠ BREAKING CHANGES
 
 * **harness:** removed the deprecated bespoke LangGraph tracing handler `create_langgraph_tracing_handler` (and its `AgentexLangGraphTracingHandler` class) from the public `agentex.lib.adk` surface. Span tracing is now derived from the canonical `StreamTaskMessage*` stream by `UnifiedEmitter` — wrap your run in the harness `*Turn` and drive `UnifiedEmitter.yield_turn` / `auto_send_turn`. The `agentex init` templates were migrated accordingly.
+* **harness:** removed the deprecated bespoke Pydantic-AI tracing handler `create_pydantic_ai_tracing_handler` (and its `AgentexPydanticAITracingHandler` class) from the public `agentex.lib.adk` surface. Span tracing is now derived from the canonical `StreamTaskMessage*` stream by `UnifiedEmitter` — wrap your run in `PydanticAITurn` and drive `UnifiedEmitter.yield_turn` / `auto_send_turn`. The `agentex init` templates were migrated accordingly.
 * **harness:** each harness now exposes exactly `_<harness>_sync.py` + `_<harness>_turn.py` under `agentex.lib.adk._modules`. The OpenAI harness `OpenAITurn` and `convert_openai_to_agentex_events` moved to `agentex.lib.adk._modules._openai_turn` / `_openai_sync`; back-compat shims remain at `agentex.lib.adk.providers._modules.{openai_turn,sync_provider}` for one release. Public facade names (`stream_pydantic_ai_events`, `stream_langgraph_events`, `emit_langgraph_messages`, etc.) are unchanged.
 
 ### Features