[feat](trx-backend): add register_builtin_backends_on for context initialization

Add register_builtin_backends_on(context: &mut RegistrationContext) function
to allow explicit backend registration on a context instead of always using globals.

This enables proper initialization sequencing where backends are registered
on a specific context that can be passed through bootstrap.

The global register_builtin_backends() still works for plugin compatibility,
delegating to the new context-based approach.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-server/trx-backend/src/lib.rs

@@ -105,7 +105,16 @@ pub fn register_backend(name: &str, factory: BackendFactory) {
     reg.factories.insert(key, factory);
 }
 
-/// Register all built-in backends enabled by features.
+/// Register all built-in backends enabled by features on a context.
+pub fn register_builtin_backends_on(context: &mut RegistrationContext) {
+    context.register_backend("dummy", dummy_factory);
+    #[cfg(feature = "ft817")]
+    context.register_backend("ft817", ft817_factory);
+    #[cfg(feature = "ft450d")]
+    context.register_backend("ft450d", ft450d_factory);
+}
+
+/// Register all built-in backends enabled by features (global, for plugin compatibility).
 pub fn register_builtin_backends() {
     register_backend("dummy", dummy_factory);
     #[cfg(feature = "ft817")]