feat(config): pass through global LiteLLM settings via a litellm: config block

[KylinMountain]

What

Adds an optional litellm: mapping to .openkb/config.yaml whose keys are passed through verbatim onto the litellm module (litellm.<key>):

litellm:
  drop_params: true     # let LiteLLM drop params a provider rejects
  num_retries: 3

Why

Closes #137. A user on a local Ollama backend hit litellm.UnsupportedParamsError because OpenKB sends parallel_tool_calls, which Ollama doesn't support. The fix LiteLLM suggests is litellm.drop_params=True, but OpenKB only forwarded model (plus the existing extra_headers/timeout whitelist) — there was no way to set it from config, only the LITELLM_DROP_PARAMS env var. This generalizes the existing per-key whitelist to a verbatim passthrough so any LiteLLM module global can be set from config.

How

• config.resolve_litellm_settings() — validates the value is a mapping, drops non-string keys, forwards values unchanged (no validation/coercion; the user owns them).
• cli._apply_litellm_settings() — setattrs each key onto litellm inside _setup_llm_key, so it covers every command and every call path routed through LiteLLM (these are module-level globals). Two guards:
  • an unknown key (typo / litellm-version mismatch) is skipped with a warning instead of silently creating a dead attribute;
  • a key whose current value is callable (e.g. litellm.completion) is refused — overwriting a function with a config scalar would brick later calls.
• Warnings go through the logger (stderr, like the sibling resolvers), not click.echo, so a typo can't corrupt piped stdout (e.g. openkb query > out.txt).
• Process-wide and sticky on purpose: settings are applied, never reset (documented in the docstring, README, and config.yaml.example).

Notes

• The currently-pinned PageIndex hardcodes litellm.drop_params=True per call during long-PDF indexing, so a user's drop_params: false won't make indexing strict — noted in the docs. (A separate upstream PageIndex change stops it from mutating the shared global.)

Tests

tests/test_config.py (resolver: mapping/non-mapping/non-string-key + caplog on the warning paths) and a new tests/test_llm_config_passthrough.py (verbatim apply, unknown-key + callable guards, sticky no-reset, env-isolated end-to-end). Full suite green.