POC: Add context scoped attributes with fallback on baggage(DO NOT MERGE)

[wrisa]

See PR, only baggage support

Changes Made

File: util/opentelemetry-util-genai/src/opentelemetry/util/genai/handler.py

Step 1 — Three new ContextVars + helper (lines 167–186)

Added _ctx_workflow_name, _ctx_agent_name, _ctx_agent_id below _current_genai_span, plus _get_ctx_attr() which reads from the ContextVar first and falls back to baggage.

Step 2 — Dual-write in _push_current_span (lines 652–664)

• For Workflow: sets baggage and calls _ctx_workflow_name.set(), storing the token on invocation._ctx_tokens.
• For AgentInvocation: sets baggage and calls .set() for both _ctx_agent_name / _ctx_agent_id, storing both tokens in invocation._ctx_tokens.

Step 3 — Token reset in _pop_current_span (lines 684–688)

After detaching the OTel context token, iterates invocation._ctx_tokens and calls ctx_var.reset(tok) for each, restoring the previous ContextVar state.

Step 4 — 6 read sites replaced

All baggage.get_baggage("workflow.name/agent.name/agent.id") calls in start_llm, start_embedding, start_retrieval, start_tool_call, start_agent, and start_step are now _get_ctx_attr(...) calls.

Baggage writes are kept (dual-write), so cross-boundary propagation via FastMCP/MCP is preserved.