Add CoreModel 2.8.0 database function support

[colemancda]

Summary

Updates the CoreModel dependency to 2.8.0 and implements the new custom database function API introduced in CoreModel PR #22.

CoreModel 2.8.0 lets predicates and sort descriptors reference custom DatabaseFunctions (arbitrary Swift closures over AttributeValues). Since MongoDB cannot execute those closures server-side, this backend mirrors the CoreData backend's strategy: fetch a native superset with function comparisons stripped from the predicate, then filter, sort, and paginate in memory.

Changes

• Package.swift — require CoreModel 2.8.0.
• Predicate.swift — the Expression.function case is untranslatable to a Mongo query, so it returns nil (falls back to the in-memory path).
• FetchRequest.swift — SortDescriptor now stores a term (.property / .function); the server-side sort document only emits .property terms.
• FunctionEvaluation.swift (new) — in-memory predicate evaluation and sorting that invoke registered functions, ported from CoreModel's CoreData backend.
• MongoDatabase.swift — MongoModelStorage now implements the new required register(function:) protocol method, keeps a function registry, and routes fetch/count/fetchID through an in-memory path when the request references a function. Also marks MongoDatabase retroactively Sendable, matching the existing pattern for MongoCollectionOptions.

Tests

• TestModel.swift — nested value types are marked Sendable (required now that Entity: Sendable).
• LegacyCodable/ — vendors CoreModel's Codable-to-ModelData bridge, which 2.8.0 moved out of the library into its own test target; restores the default Entity conformance for the hand-written Codable test models.

The library builds cleanly and the test target compiles. The integration tests require a live MongoDB at localhost:27017 (provided via Docker in CI) and were not executed locally.