* Decode TSession.userRoles with backward compatibility

Sessions persisted before #76 added userRoles lack the key, so the
synthesized Codable decoder throws .keyNotFound. In TidepoolService that
kills init?(rawState:), which drops the saved service entry and forces
the user to re-authenticate on the next upgrade.

Provide a custom init(from:) that uses decodeIfPresent for userRoles
defaulting to []. Keeps the public API unchanged (userRoles stays
non-optional) and adds a backward-compat test.

* ci: bump Xcode to 16.4 on Apple Silicon resource class

CircleCI's m4pro.medium resource class does not support Xcode 14.1.0.
Use Xcode 16.4 and drop the OS pin on the simulator destination.

* tests: compare XCTAssertCodableAsJSON semantically

Newer Foundation emits Doubles at full IEEE-754 precision (e.g. 2.34 ->
2.3399...) and key order can differ between JSONEncoder and
JSONSerialization output -- both of which broke the previous raw string
comparison even though the data is equivalent. Parse both sides back to
Foundation objects and compare via NSObject equality, which handles
nested values, key ordering, and number equivalence by value.

Brings 46 pre-existing test failures back to green on current SDKs (and
preserves the encode/decode round-trip assertions).