Get rid of exported global variables and instead make them a part of the "huma.API" struct

About to implement a (more-strict) golangci-lint config for this repo, I noticed there are quite a few global variables inside the project.
Just to name some of the more relevant ones:

```
adapters/humago/humago.go:19:5: MultipartMaxMemory is a global variable (gochecknoglobals)
var MultipartMaxMemory int64 = 8 * 1024

defaults.go:16:5: DefaultJSONFormat is a global variable (gochecknoglobals)
var DefaultJSONFormat = Format{

error.go:251:5: NewErrorWithContext is a global variable (gochecknoglobals)
var NewErrorWithContext = func(_ Context, status int, msg string, errs ...error) StatusError {

huma.go:1977:5: GenerateOperationID is a global variable (gochecknoglobals)
var GenerateOperationID = func(method, path string, response any) string {

schema.go:29:5: DefaultArrayNullable is a global variable (gochecknoglobals)
var DefaultArrayNullable = true
```

While it is definitely nice and helpful to provide a way to configure these (default) settings, they all depend on a single, shared global state.

In order to help mitigate problems which arise when using multiple Huma apps (`huma.API` contexts) in the same project, race conditions, etc., those settings should instead be a part of the `huma.API` context itself.

For example, I could imagine configuring the default Huma error handler (currently the global variable `huma.NewErrorWithContext`) as follows:

```go
humago.New(
mux,
huma.DefaultConfig("API", "1.0.0").
WithErrorHandler(func(ctx huma.Context, status int, msg string, errs ...error) huma.StatusError {
...
}),
)
```

```patch
diff --git a/api.go b/api.go
index e99bc43948bc7859a06b8ecf9dd5cd83300c65b9..e2c86418d2058cd3edcce88131c359a614a66ff0 100644
--- a/api.go
+++ b/api.go
@@ -203,8 +203,12 @@ type Config struct {
// for example if you need access to the path settings that may be changed
// by the user after the defaults have been set.
CreateHooks []func(Config) Config
+
+ ErrorHandler ErrorHandler
}

+type ErrorHandler func(ctx Context, status int, msg string, errs ...error) StatusError
+
// API represents a Huma API wrapping a specific router.
type API interface {
// Adapter returns the router adapter for this API, providing a generic
@@ -265,6 +269,9 @@ type api struct {
formatKeys []string
transformers []Transformer
middlewares Middlewares
+
+ // NOTE: Ensure errorHandler is an unexported property
+ errorHandler ErrorHandler
}

func (a *api) Adapter() Adapter {
@@ -400,6 +407,11 @@ func NewAPI(config Config, a Adapter) API {
newAPI.formatKeys = append(newAPI.formatKeys, k)
}

+ if config.ErrorHandler == nil {
+ config.ErrorHandler = defaultErrorHandler
+ }
+ newAPI.errorHandler = config.ErrorHandler
+
if config.OpenAPIPath != "" {
var specJSON []byte
a.Handle(&Operation{
```

Start by reviewing the listed globals in adapters/humago/humago.go, defaults.go, error.go, huma.go, and schema.go, then trace how API and Config are constructed in api.go. Determine the complete set of exported global settings and how each belongs to an individual huma.API. Done means separate API contexts no longer share those configurable settings and the stricter golangci-lint configuration no longer reports them.

Written by the indexing model from the issue text.