When the primary auxiliary compression model fails the minimum
context-length check (64K floor), iterate through the user's
fallback_providers chain instead of raising ValueError and
killing the session.
- check_compression_model_feasibility: catch ValueError from
MINIMUM_CONTEXT_LENGTH rejection, try each fallback provider
in order, store the first suitable one as
_compression_fallback on the ContextCompressor
- ContextCompressor._generate_summary: when _compression_fallback
is set, override main_runtime (provider/model/base_url/api_key)
so summaries route to the fallback provider
- When all fallbacks are exhausted, emit a clear warning and
continue without summaries (same as 'no auxiliary LLM' path)
Fixes the session-killing crash when switching to a model whose
provider's auto-detected compression model has <64K context.
