feat: add Kilo Code support and model selection to ACP configurations

[vvlrff]

Why are these changes needed?

Two gaps in the ACP integration:

1. ACPConfig.model was response metadata only — there was no way to actually
   select the CLI agent's model from AG2.
2. Kilo Code was not supported.

This PR:

• Applies ACPConfig.model at session start via ACP session/set_config_option
  when the agent advertises a model picker in session/new (Claude Code,
  OpenCode and Kilo Code all do). A value equal to the agent's current model is
  not re-sent; a model not offered by the agent raises ValueError; agents
  without a model option keep the previous metadata-only behavior.
• Adds KiloCodeConfig — an ACPConfig preset launching kilo acp. model
  should always be set explicitly, since a fresh Kilo ACP session may default
  to an unsuitable model.
• Extends the fake_acp_config testing double with config_options /
  config_option_calls seams, so e2e tests cover model selection through the
  public API only.
• Updates the CLI agents user guide (website/docs/user-guide/cli_agents.mdx).

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.

AI assistance

• I understand the changes in this PR and can explain them in my own words.
• I have verified that the PR description accurately reflects the actual diff.
• If AI assistance was used, I reviewed, tested, and validated the generated code/text before submitting.

[codecov]

Codecov Report

❌ Patch coverage is 84.21053% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ag2/acp/session.py	77.77%	4 Missing and 2 partials ⚠️

Files with missing lines	Coverage Δ
ag2/acp/__init__.py	100.00% <100.00%> (ø)
ag2/acp/client.py	81.08% <ø> (ø)
ag2/acp/config.py	100.00% <100.00%> (ø)
ag2/acp/testing.py	100.00% <100.00%> (ø)
ag2/acp/session.py	86.36% <77.77%> (-1.33%)	⬇️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.