feat(coglet): restore Scope.context for per-prediction context (#2853)
* feat(coglet): restore Scope.context for per-prediction context
Re-implement the context property on coglet's Scope that was lost when
the Python server was replaced by the Rust coglet in #2714. The original
feature (#2330) allowed predictors to access per-prediction context via
current_scope().context.
Thread context (HashMap<String, String>) from the HTTP request body
through the full stack: PredictionRequest -> service.predict ->
SlotRequest::Predict -> worker -> ScopeGuard -> Scope.context getter.
Closes #2852
* test: add integration test for Scope.context
* fix: correct Python bool casing in scope_context test assertion