refactor: deprecate LLMConfig contextmanager, .current, .default

[Lancetnik]

1. deprecate LLMConfig.current, LLMConfig.default
2. deprecate LLMConfig(config_list=[...])
3. deprecate LLMConfig(**options)
4. Add LLMConfig(*configs) API
5. update all documentation, notebooks, tests, codebase deprecated features usages

All deprecations are scheduled for removal in v0.11.0 release

As a part of #1946
Closes #1412

• 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.

[Lancetnik]

@marklysze probably, you can review just an autogen/ directory changes. All other stuff is just a deprecated syntax usages updates (in notebooks, documentation, tests, etc)

[qingyun-wu]

@marklysze will you review this PR?

[priyansh4320]

I have a concern about Doc Markdown breakage. Can you please confirm that markdowns aren't breaking?, due to changes , with a couple of screenshots in the description?
also can you please confirm if you have tested the docs by building and serving docs locally?

[Lancetnik]

I have a concern about Doc Markdown breakage. Can you please confirm that markdowns aren't breaking?, due to changes , with a couple of screenshots in the description? also can you please confirm if you have tested the docs by building and serving docs locally?

I didn't check all pages, but everything seems fine

[joggrbot]

📝 Documentation Analysis

All docs are up to date! 🎉

---

✅ Latest commit analyzed: aa626c3 | Powered by Joggr

[marklysze]

Thanks @Lancetnik, I'm good with this.

@sonichi as it's changing core LLMConfig please feel free to review if you want. Let's aim to merge by end of this week if no blockers.

[priyansh4320]

hey, @Lancetnik bro I am approving this as well

[priyansh4320]

requesting changes. this branch needs a rebase.

[codecov]

Codecov Report

❌ Patch coverage is 75.93985% with 32 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
autogen/mcp/mcp_proxy/operation_grouping.py	0.00%	24 Missing ⚠️
autogen/interop/pydantic_ai/pydantic_ai.py	0.00%	6 Missing ⚠️
autogen/llm_config/config.py	95.65%	1 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
autogen/agentchat/conversable_agent.py	63.69% <100.00%> (-1.69%)	⬇️
autogen/llm_config/entry.py	89.79% <100.00%> (-1.96%)	⬇️
autogen/llm_config/utils.py	100.00% <100.00%> (ø)
autogen/oai/client.py	56.56% <100.00%> (-3.83%)	⬇️
autogen/oai/openai_utils.py	62.97% <100.00%> (-2.91%)	⬇️
autogen/llm_config/config.py	91.93% <95.65%> (+3.84%)	⬆️
autogen/interop/pydantic_ai/pydantic_ai.py	41.79% <0.00%> (-1.29%)	⬇️
autogen/mcp/mcp_proxy/operation_grouping.py	35.21% <0.00%> (+1.42%)	⬆️

... and 34 files with indirect coverage changes

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