fix: run middleware added by group modifiers

[stareezy-1]

Summary

Fixes #804

Problem

Middleware appended to op.Middlewares inside a group modifier never runs at request time — even though it does appear in the generated OpenAPI document.

Root cause: PrefixModifier copies the operation for fan-out (so multiple prefixes can be documented and registered), which means operation modifiers mutate a copy. huma.Register builds the runtime handler chain from the original operation's op.Middlewares:

a.Handle(&op, api.Middlewares().Handler(op.Middlewares.Handler(...)))

The original op never sees the modifier's changes, so the middleware is missing from the chain. Tags/Security/Errors worked because they only matter for the document, which uses the modified copy.

Reproduction: register an operation on a group whose UseSimpleModifier appends a middleware, send a request — the middleware never runs.

Fix

• Register (group path): skip the op.Middlewares pre-wrap — the original op's list is stale by definition; the group adapter handles it.
• groupAdapter.Handle: wrap the handler with the final post-modifier operation's op.Middlewares before delegating to the underlying adapter.

This runs modifier-added middleware exactly once and avoids double-running directly-set operation middlewares (which would happen if both the original and the copy were wrapped).

Tests

TestGroupModifierMiddlewareRuns registers an operation in a group whose modifier appends a middleware setting a response header, then asserts:

Assertion	Verifies
Request returns 200	handler still executes
X-Modifier-Middleware: ran header present	modifier middleware actually ran

All existing tests continue to pass (go test -race ./...).