fix: respect contextLength from YAML model config (#11807)

* fix: respect contextLength from YAML model config

Three fixes for contextLength not being respected:

1. Add contextLength to YAML model schema at the model level (not just
   inside defaultCompletionOptions), matching JSON config behavior
2. Read contextLength from both model-level and defaultCompletionOptions,
   with model-level taking precedence
3. Fix GUI selector fallback using DEFAULT_MAX_TOKENS (4096) instead of
   DEFAULT_CONTEXT_LENGTH (32768)

Closes #4638
Closes #4304

* fix: reduce complexity in modelConfigToBaseLLM

Replace find callbacks with includes() to offset complexity from
contextLength nullish coalescing.

* docs: document contextLength at model level in config.yaml reference

* docs: consistently show contextLength in defaultCompletionOptions for YAML

YAML examples use defaultCompletionOptions.contextLength, JSON examples
use model-level contextLength. Also fixes ollama guide using the wrong
key name (completionOptions -> defaultCompletionOptions).

* refactor: extract env options into helper to reduce complexity

Extract the repetitive env-to-options mapping from modelConfigToBaseLLM
into a separate applyEnvOptions function, bringing complexity well under
the limit of 36.