chore: delete dead func_inspection.py, document jsonschema validation removal

- Remove func_inspection.py which is no longer imported after decorator removal
- Add migration note about jsonschema input/output validation being removed
  from the lowlevel Server's call_tool handler with no built-in replacement

Author: maxisbey

docs/migration.md

@@ -486,6 +486,7 @@ server = Server(
 
 - Handlers receive `(ctx, params)` instead of the full request object or unpacked arguments. `ctx` is a `RequestContext` with `session`, `lifespan_context`, and `experimental` fields (plus `request_id`, `meta`, etc. for request handlers). `params` is the typed request params object.
 - Handlers return the full result type (e.g. `ListToolsResult`) rather than unwrapped values (e.g. `list[Tool]`).
+- The automatic `jsonschema` input/output validation that the old `call_tool()` decorator performed has been removed. There is no built-in replacement — if you relied on schema validation in the lowlevel server, you will need to validate inputs yourself in your handler.
 
 **Notification handlers:**