Fix JSR publishing by removing MockContext export

The MockContext class contained complex type dependencies that caused
JSR's type analyzer to hang indefinitely during the "processing" stage.
This commit removes MockContext from the public API exports.

Users who were using MockContext directly should migrate to using the
createContext(), createRequestContext(), or createInboxContext() helper
functions instead.

Breaking change: MockContext is no longer exported from @fedify/testing

Fixes https://github.com/fedify-dev/fedify/issues/468



Co-Authored-By: Claude <noreply@anthropic.com>