codegen: check message and enum names against service-derived items #330

In connectrpc-build output a package's messages and enums share a Rust module with its service stubs. check_module_collisions (#280) compares the identifiers derived from one service with those derived from another, and does not look at message or enum names. A message named like a service-derived item is accepted at generation time, and the generated code then fails to compile:

service Greet {
  rpc Say(SayRequest) returns (SayResponse);
}
message GreetClient {}

error[E0428]: the name `GreetClient` is defined multiple times

The types a service Greet derives are Greet, GreetExt, GreetServer, GreetClient and GreetRegisterMarker. Its constants are values, which a message or enum does not collide with. The Owned<Message>View aliases are types in the same module, so message OwnedSayRequestView collides with the alias for SayRequest; the alias check from #77 compares aliases with each other only.

The split layout (buf generate with separate proto and connect trees) is not affected, because messages are in a different module there.

Proposed: before the per-service pass, record the package's top-level message and enum names in BatchState::module_idents, so that the existing check reports the pair with both proto names, as it does for two services. Only the unified layout needs it.

#329 documents the limit in docs/guide.md; the fix removes that sentence.