Fix JSR publishing hang by removing @opentelemetry/api dependency

Removed all dependencies on @opentelemetry/api package from @fedify/testing
to resolve JSR publishing issues. When @opentelemetry/api types were imported
alongside ResourceDescriptor from @fedify/fedify/webfinger, JSR's type graph
analyzer would hang indefinitely during the "processing" stage.

Changes:

- Removed @opentelemetry/api from package.json dependencies
- Removed @opentelemetry/api from deno.json imports
- Removed @opentelemetry/api from tsdown.config.ts external list
- Replaced TracerProvider type with `any` in mock.ts and context.ts
- Implemented no-op tracer provider inline to avoid external dependency
- Added detailed comments explaining the workaround
- Added deno-lint-ignore-file directives for no-explicit-any
- Moved createRequestContext and createInboxContext from context.ts to mock.ts
- Updated CHANGES.md with fix details

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



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