Make the uplink match the written protocol, and test that it does

Adds tests/uplink_protocol.rs, the gateway half of a contract test whose
other half is continuum-backend/tests/edge_protocol.test.ts. It checks that
every message the backend sends is one this gateway can read, and that what
this gateway writes is what the schema says it writes.

It found two things immediately.

Telemetry was serializing empty fields as explicit nulls. serde emits
`"job_name": null` for an empty Option unless told otherwise, and the
backend's validator treats a missing key and an explicit null as different
things - so every batch containing an idle printer, which by definition has
no job name, would have been rejected whole. Every optional field now has
skip_serializing_if, confirmed against a live capture: an idle P1P's
snapshot has no nulls in it at all.

ServerMessage had no variant for the handshake being rejected. A bad or
expired token would have arrived as "unparseable message from cloud,
ignoring" and then been retried every five seconds forever, which in the
logs is indistinguishable from a network fault. HelloError is now a real
variant, logged at error level naming the env var to go fix, and it stops
the connection attempt instead of looping on it.

Also exposes config and uplink from lib.rs so the test can reach them, same
reason fleet and printer are already there.

Verified live against a P1P and an H2C for the whole run, including the H2C
finishing its print: RUNNING 98% -> FINISH 100% layer 50/50 with the nozzle
cooling 84C -> 76C, all of it arriving through the uplink, no warnings.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/lib.rs

@@ -3,5 +3,7 @@
 //! `examples/`) and future integration tests can `use continuum_proxy::...`
 //! without duplicating module declarations.
 
+pub mod config;
 pub mod fleet;
 pub mod printer;
+pub mod uplink;