Add support for patternProperties

[ChrisJr404]

Closes #1102.

Adds a PatternProperties field to huma.Schema so object schemas can validate properties whose names match a regular expression, as described by the JSON Schema / OpenAPI 3.1 patternProperties keyword. This lets you express things like "any key matching ^str_[a-z]+$ must be a string" that weren't previously possible.

Behaviour

• PatternProperties map[string]*Schema marshals into the generated OpenAPI document as patternProperties.
• Patterns are compiled once in PrecomputeMessages (stored next to the existing pattern regex) and iterated in sorted order, so validation stays allocation-free on the happy path and error ordering is deterministic.
• During validation, each instance property is checked against every patternProperties schema whose regex matches its name (both the map[string]any and map[any]any paths).
• A property matched by a pattern is no longer treated as an additional property, so it's exempt from both additionalProperties: false and the additionalProperties: <schema> cases — matching the spec.
• autopatch carries the field through to generated PATCH schemas.

Tests

• TestSchemaPatternProperties covers marshaling.
• Eight TestValidate cases cover value type/constraint failures, the additionalProperties interaction (bool and schema forms), and the map[any]any path.

Kept the scope small per the note in #1040 about smaller PRs being easier to review. Happy to adjust the field naming or the additionalProperties interaction if you'd prefer a different approach.