fix: add keepAlive to YAML completion options schema (#11801)

* fix: add keepAlive to YAML completion options schema

The keepAlive property was present in the JSON config schema but missing
from the YAML config schema, causing it to be silently stripped during
zod validation when users configured it under defaultCompletionOptions.

Closes #4671

* Document keepAlive option in YAML config reference

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Co-authored-by: continue[bot] <230936708+continue[bot]@users.noreply.github.com>