feat: integrate async for parallel expansion in reasoning agent

[Hk669]

Why are these changes needed?

Related issue number

#246

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/latest/docs/contributor-guide/documentation/ to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

Attention: Patch coverage is 5.64103% with 184 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...n/agents/experimental/reasoning/reasoning_agent.py	5.64%	183 Missing and 1 partial ⚠️

❗ There is a different number of reports uploaded between BASE (dd69502) and HEAD (65ae518). Click for more details.

HEAD has 1290 uploads less than BASE

Flag	BASE (dd69502)	HEAD (65ae518)
3.10	106	0
ubuntu-latest	129	1
optional-deps	175	0
commsagent-slack	9	0
3.13	91	0
commsagent-telegram	9	0
browser-use	3	0
macos-latest	114	0
core-without-llm	14	1
3.11	48	1
commsagent-discord	9	0
3.12	36	0
rag	7	0
3.9	86	0
windows-latest	124	0
jupyter-executor	9	0
interop-crewai	9	0
interop-langchain	9	0
retrievechat-pgvector	10	0
crawl4ai	9	0
graph-rag-falkor-db	6	0
twilio	9	0
mcp	9	0
interop	9	0
docs	6	0
retrievechat-mongodb	10	0
websockets	9	0
retrievechat-qdrant	14	0
agent-eval	1	0
retrievechat	15	0
anthropic	15	0
llama-index-agent	3	0
long-context	3	0
retrievechat-couchbase	3	0
teachable	3	0
gpt-assistant-agent	3	0
interop-pydantic-ai	9	0
google-api	9	0
together	14	0
cerebras	14	0
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websurfer	15	0
mistral	14	0
gemini	14	0
cohere	15	0
groq	14	0
ollama	14	0
swarm	14	0
bedrock	14	0

Files with missing lines	Coverage Δ
...n/agents/experimental/reasoning/reasoning_agent.py	29.62% <5.64%> (-11.53%)	⬇️

... and 57 files with indirect coverage changes

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