Fix JSR publishing hang by removing function overloads

The @fedify/testing package was hanging indefinitely during JSR's type
analysis stage. After extensive binary search testing, the root cause
was identified as TypeScript function overload signatures in MockContext
and MockFederation classes.

Removed all function overloads from the following methods:

- MockContext.sendActivity (had 5 overload signatures)
- MockContext.lookupNodeInfo (had 2 overload signatures)
- MockContext.getDocumentLoader (had 2 overload signatures)
- MockContext.getInboxUri (had 2 overload signatures)
- MockFederation.createContext (had 2 overload signatures)

All overloaded methods have been simplified to single signatures using
`any` types where necessary. This is a workaround for a bug in JSR's
type analyzer that struggles with certain function overload patterns.

Also removed unused imports that resulted from simplifying the type
signatures.

Fixes https://github.com/fedify-dev/fedify/issues/464



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