bug: Model.from_resource() does not cast fields to their declared schema types (created_at is plain str, not DateTime) #37

Summary

Model.from_resource() constructs models by passing the raw JSON dict straight
into the dataclass constructor — it never casts values to the declared field
types. Fields declared as schema types (e.g. created_at: schema.DateTime,
id: schema.CampusID) therefore come back as plain builtins (str) after
parsing an API response, violating the model's own type contract.

Current behaviour

# campus/model/base.py
@classmethod
def from_resource(cls, resource):
    return cls(**{
        field.name: resource[field.name]
        for field in cls.fields().values()
        if field.metadata.get("resource", True)
    })

json.loads produces plain str; nothing wraps it, so:

c = Client.from_resource({"id": ..., "created_at": "2026-09-29T10:00:00+00:00", ...})
type(c.created_at)          # <class 'str'>, not schema.DateTime
isinstance(c.created_at, schema.DateTime)  # False
c.created_at.to_datetime()  # AttributeError — emulated method unavailable

This matters because DateTime is deliberately a str subclass with emulated
datetime helpers (to_datetime(), utcnow(), ...) — consumers of parsed models
currently get none of them and cannot distinguish a parsed model from a plain dict.

Observed impact

campus-cli assumed created_at was a datetime and called .isoformat() on it —
every live client new / client get crashed with
'str' object has no attribute 'isoformat' (fixed defensively on the CLI side in
campus-cli PR #12). The declared-but-unenforced field type is what made that bug
easy to write and hard to spot.

Proposed fix

In from_resource() (or a field-level coercion hook), cast values to their
declared schema types when the declared type is a schema class — e.g.
cls(value) for DateTime/CampusID string subclasses — leaving genuinely
primitive fields untouched. Alternatively, if coercion is out of scope, document
prominently that from_resource models carry plain primitives so the annotations
aren't read as runtime guarantees.

Acceptance criteria

• Client.from_resource(...).created_at is an instance of schema.DateTime
  and supports the emulated helpers (to_datetime())
• Same treatment for other schema-typed fields (e.g. CampusID)
• Existing behaviour for plain-primitive fields unchanged
• Tests cover a parsed resource round-trip asserting field types