backend(audio-cpp): make the missing-switch-case diagnostic fatal

unsupported_surface() switches UnsupportedRpc onto the table row that explains
it, with no default label, so -Wswitch reports an enumerator nobody handled. As
a warning that is not enough: adding a sixth enumerator and building the shipping
target gives exit 0, a binary and one warning, and the trailing
`return surfaces[0];` then answers the new RPC with AudioEncode's codec reason.
That is a confident, specific and false statement about audio.cpp on the wire, on
the one code path whose entire job is to be truthful about what this backend
cannot do, and it is worse than the runtime fallback it replaced, which at least
named itself as a bug in this file.

capability_routing.cpp therefore joins loaded_model.cpp on the existing
-Werror=switch pin, whose comment already made this argument for the engine enum.
The comment now covers both files. The pin stays per-file rather than
project-wide because upstream's own ace_step/vae_decoder.cpp has unhandled
-Wswitch cases of its own.

Verified: a sixth enumerator now fails `make grpc-server` with exit 2 and no
binary; appending a 14th VoiceTaskKind upstream still fails loaded_model.cpp, so
the two pins fire independently; both reverted clean.

Assisted-by: Claude:claude-opus-5 [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

backend/cpp/audio-cpp/CMakeLists.txt

@@ -138,17 +138,32 @@ add_executable(${TARGET}
     live_watchdog.cpp
 )
 
+# Two files carry a switch over an enum with no `default:` label, deliberately,
+# so that -Wswitch reports an enumerator nobody handled. -Wswitch is only a
+# warning by default, and a warning in a 600-file build log is a warning nobody
+# reads, so it is promoted to an error on exactly these two translation units.
+# Not project-wide: upstream's own sources are not held to this, and they are
+# where the churn is.
+#
 # loaded_model.cpp mirrors engine::runtime::VoiceTaskKind onto its own Task enum.
 # Its static_asserts catch an insertion or a reorder, but an enumerator APPENDED
 # after the last one shifts no value, so no assertion can see it. What does see
-# it is from_engine_task's switch over the engine enum, which carries no
-# `default:` label. -Wswitch is only a warning by default, and a warning in a
-# 600-file build log is a warning nobody reads, so promote it here: this is the
-# difference between a build failure and a backend that silently runs the wrong
-# task.
+# it is from_engine_task's switch over the engine enum. This is the difference
+# between a build failure and a backend that silently runs the wrong task.
+#
+# capability_routing.cpp's unsupported_surface() switches UnsupportedRpc onto the
+# row of unsupported_surfaces() that explains it. Left as a warning, a sixth
+# enumerator added without a row BUILDS AND SHIPS, and its trailing
+# `return surfaces[0];` then answers the new RPC with AudioEncode's codec reason:
+# a confident, specific and false statement about audio.cpp, on the wire, on the
+# one code path whose entire job is to be truthful about what this backend
+# cannot do. Verified rather than assumed: adding a sixth enumerator and building
+# the shipping target produced exit 0, a binary, and one warning. A compile-time
+# check is the better trade than the runtime fallback it replaced only if it is
+# fatal, so here it is fatal.
 if(NOT MSVC)
-    set_source_files_properties(loaded_model.cpp PROPERTIES
-        COMPILE_OPTIONS "-Werror=switch")
+    set_source_files_properties(loaded_model.cpp capability_routing.cpp
+        PROPERTIES COMPILE_OPTIONS "-Werror=switch")
 endif()
 
 target_include_directories(${TARGET} PRIVATE