RBS: Narrow type signatures for props parameters

Improve type precision for props parameters in RBS signatures by
changing from `untyped` to `Hash[Symbol, untyped] | String`.

This change:
- Provides better type checking for props parameters
- Accepts both common formats (Hash or JSON string)
- Improves IDE support and autocomplete
- Helps catch type errors earlier

Updated signatures:
- helper.rbs: sanitized_props_string props parameter
- controller.rbs: redux_store props parameter

Fixes #1951

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

sig/react_on_rails/controller.rbs

@@ -5,7 +5,7 @@ module ReactOnRails
 
     def redux_store: (
       String store_name,
-      ?props: untyped,
+      ?props: Hash[Symbol, untyped] | String,
       ?immediate_hydration: bool
     ) -> void
 

sig/react_on_rails/helper.rbs

@@ -21,7 +21,7 @@ module ReactOnRails
     # Returns html_safe string (ActiveSupport::SafeBuffer)
     def redux_store: (
       String store_name,
-      ?Hash[Symbol, untyped] props
+      ?Hash[Symbol, untyped] | String props
     ) -> safe_buffer
 
     # Returns html_safe string (ActiveSupport::SafeBuffer)
@@ -33,7 +33,7 @@ module ReactOnRails
       ?Hash[Symbol, untyped] options
     ) -> safe_buffer
 
-    def sanitized_props_string: (untyped props) -> String
+    def sanitized_props_string: (Hash[Symbol, untyped] | String props) -> String
 
     def rails_context: (
       ?server_side: bool