test(model): pin from_resource schema-type coercion (issue #37)

[nycomp]

Summary

Issue #37 reported that Model.from_resource() returned plain builtins for fields declared as schema types (created_at as plain str instead of schema.DateTime), which is what let campus-cli call .isoformat() on a string and crash on every live client new / client get.

The coercion fix itself has already landed upstream in campus and is active in this repo's dependency pin — no production code change is needed here:

• campus dfb7d0c (fix(model): deserialize init=False and nested fields in from_resource) added str→str-subclass coercion and nested-model deserialization to Model.from_resource(), with its own unit tests
• campus ac9dfdc mirrored the coercion in from_storage()
• both are included in campus-suite 9e37b85, which this repo's poetry.lock and venv already pin (PR chore(deps): bump campus-suite to weekly head 9e37b85 (campus #658 alignment check) #46)

Verified against the pinned campus-suite: Client.from_resource(...).created_at is schema.DateTime with working to_datetime(), id/email coerce to CampusID/Email, plain-primitive fields (str/bool/int) are untouched, and to_resource() round-trips JSON-serialisably.

What this PR does

Adds tests/unit/test_model_field_types.py pinning the field-type contract at this repo's boundary, following the same pattern as test_oauth_token_contract.py:

• schema-typed fields (DateTime, CampusID, Email) are coerced on parsed resources, including the nested OAuthToken in UserCredentials and models returned by auth.clients.new() / .get() — the exact paths campus-cli consumes
• emulated helpers (to_datetime(), Email.domain) are available on coerced fields
• plain-primitive fields stay plain builtins; parsed models round-trip

If a future campus-suite bump regresses the coercion, CI fails here instead of in campus-cli.

Fixes #37

Test plan

• .venv/Scripts/python.exe -m pytest: 89 passed (80 existing + 9 new)