fix: document error codes added by group modifiers

[stareezy-1]

Summary

Fixes #878

Problem

When a group modifier appends to op.Errors, the generated OpenAPI document does not include the new error responses — while Tags, Security, and Middlewares changes from the same modifier are reflected correctly.

Root cause: huma.Register called defineErrors (which consumes op.Errors into op.Responses) before invoking OperationDocumenter, where group modifiers run. The other fields are read directly from the operation by AddOperation after the modifiers, which is why they worked.

Reproduction:

internalApi.UseSimpleModifier(func(op *huma.Operation) {
	op.Tags = append(op.Tags, "Internal")        // appears in the doc ✓
	op.Errors = append(op.Errors, 401)           // missing from the doc ✗
})

Fix

Move the error definition to the innermost document step — after all group modifiers have run and immediately before AddOperation:

• Register (plain API path): call defineOperationErrors right before AddOperation.
• Group.DocumentOperation: call it at the end of the modifier chain, before delegating or adding.
• New defineOperationErrors helper preserves the previous 422/500 auto-append behavior, using the documented operation state (op.RequestBody, op.Parameters) as the equivalent condition.

Tests

TestGroupModifierErrorsDocumented registers an operation in a group whose modifier adds 401 and asserts:

Response	Without fix	With fix
401 (modifier-added)	missing	present
422 / 500 (auto-appended)	present	present
Tags from modifier	present	present

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