Add support for `patternProperties`

Hey @danielgtaylor @wolveix

Thanks for maintaining Huma.

It would be nice to support `patternProperties` at some point.

For example
```go
type PatternValues map[string]any

func (v PatternValues) Schema(r huma.Registry) *huma.Schema {
strMin, strMax := 3, 8
intMin, intMax := 1.0, 100.0

return &huma.Schema{
Type: huma.TypeObject,
PatternProperties: map[string]*huma.Schema{
"^str_[a-z]{3,8}$": {
Type: huma.TypeString,
MinLength: &strMin,
MaxLength: &strMax,
Enum: []any{"alpha", "bravo", "charlie"},
},
"^count_[0-9]{2}$": {
Type: huma.TypeInteger,
Minimum: &intMin,
Maximum: &intMax,
},
},
AdditionalProperties: false,
}
}

var _ huma.SchemaProvider = PatternValues{}

type PatternInput struct {
Body struct {
Values PatternValues `json:"values"`
}
}
```

would have the following OAS
```yaml
...
PatternInputBody:
additionalProperties: false
properties:
$schema:
description: A URL to the JSON Schema for this object.
examples:
- https://example.com/schemas/PatternInputBody.json
format: uri
readOnly: true
type: string
values:
additionalProperties: false
patternProperties:
^count_[0-9]{2}$:
maximum: 100
minimum: 1
type: integer
^str_[a-z]{3,8}$:
enum:
- alpha
- bravo
- charlie
maxLength: 8
minLength: 3
type: string
type: object
required:
- values
type: object
...
```

Validation results

| Request body | Expected validation | Failure reason |
|---|---|---|
| `{"values":{"str_name":"alpha","count_01":42}}` | `true` | |
| `{"values":{"count_01":"not an integer"}}` | `false` | `count_01` matches `^count_[0-9]{2}$`, but value is not an integer |
| `{"values":{"str_bad":"zo"}}` | `false` | `str_bad` matches `^str_[a-z]{3,8}$`, but value length is less than `3` |
| `{"values":{"str_name":"delta"}}` | `false` | `str_name` matches `^str_[a-z]{3,8}$`, but value is not in enum `alpha`, `bravo`, `charlie` |
| `{"values":{"count_99":101}}` | `false` | `count_99` matches `^count_[0-9]{2}$`, but value is greater than `100` |
| `{"values":{"name":"alice"}}` | `false` | `name` does not match any allowed pattern and `additionalProperties` is `false` |

Questions
- What do you think about supporting it?
- Would you be open to a PR for it? I have a local prototype, but I’d like to align on the scope first.

Start by tracing how the SchemaProvider example produces OpenAPI schemas and how object validation handles PatternProperties and AdditionalProperties; no files or tests are named in the issue. Done means patternProperties is emitted in the shown OAS and the six listed request bodies produce the stated validation results.

Written by the indexing model from the issue text.