extension/llm/server: document pi integration

[mergennachin]

Add an operational recipe to the server README for pointing pi (or any
OpenAI-compatible harness) at the ExecuTorch server for local tool-use: the
launch command, useful flags (--no-think / --max-context /
--allow-chatml-fallback), client base_url/model/api_key settings, the supported
chat-completions + Hermes/Qwen tool-call contract (only tool_choice
auto/none/unset; response_format/logprobs/top_p!=1/seed rejected), and
reliability guidance. Docs only; no runtime or dependency changes.

Part of #20001

[mergennachin]

Stack from ghstack (oldest at bottom):

• Qwen3.5-MoE CUDA V2 foundation: one model, many isolated sessions #20117
• examples/models/qwen3_5_moe: CUDA Engine/Session adapter + OpenAI serving #20043
• -> extension/llm/server: document pi integration #19999
• extension/llm/server: worker-based OpenAI-compatible HTTP server #19994
• extension/llm/server: serving foundations (schemas, errors, templating, tools) #19993
• extension/llm/runner: Engine/Session C++ core + token-step primitives #19991

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19999

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 3 New Failures, 1 Unrelated Failure

As of commit 83986e5 with merge base eeb0646 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 80f721ddf5514de7b00320a4b923b93d630d5d9000648f991dc907d89c22897a /exec failed with exit code 1
• pull / unittest / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 490b345dcbe8627d2ebdc7f14bc635bef5a24d7da00a5add94de5043ffb079d5 /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-editable / macos / macos-job (gh) (detected as infra flaky with no log or failing log classifier)

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