Wire up six unregistered functions

[pigri]

Summary

Six functions were implemented in the fork but not actually usable. Five had complete implementations (with tests) sitting in engine/src/functions/ that were never declared in mod.rs — so they were never compiled, exported, or registered. ends_with was exported from the engine but had no FFI dispatch arm.

This wires all of them in.

Function	Gap before	FFI name
upper	orphaned file	upper
to_string	orphaned file	to_string
remove_query_args	orphaned file	remove_query_args
json_lookup_string	orphaned file	lookup_json_string
json_lookup_integer	orphaned file	lookup_json_integer
ends_with	exported, not FFI-registered	ends_with

The JSON lookups register under Cloudflare's canonical names (lookup_json_string / lookup_json_integer), matching their own test names; the modules were just named in reverse.

Changes

• Declare + export the five modules in functions/mod.rs and engine/src/lib.rs.
• Register all six in the FFI add_function dispatch (ffi/src/lib.rs).
• Promote serde_json from a dev-dependency to a regular dependency of the engine — the JSON lookup implementations use it at runtime (non-test code), so the crate failed to build outside cargo test until this change.
• Drop two now-unused test-only imports surfaced once these files started compiling.

Test plan

• RUSTFLAGS=-D warnings cargo build --all-targets clean
• cargo clippy --all-targets -- -D clippy::all clean
• cargo +nightly fmt --check clean
• cargo test — all pass (engine lib 210 → 238, the +28 being the newly compiled functions' tests)