fix(themes): coerce the engine-theme dict params from JSON strings

Container params carry JSON_STRING_COERCION so a stringified dict from an
MCP client is parsed rather than rejected. The two new engine-theme dict
params, value and expected_current, were missing it, which
test_container_param_coercion_complete pins repo-wide.

Caught by CI's unit leg -- the targeted runs I had been doing did not
include that contract test.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012Nm7tyA1nfxNCWFXaR3AxV

Author: kingpanther13

src/ha_mcp/tools/tools_themes.py

@@ -22,7 +22,11 @@
     raise_tool_error,
     register_tool_methods,
 )
-from .util_helpers import summarize_theme_listing, websocket_error_message
+from .util_helpers import (
+    JSON_STRING_COERCION,
+    summarize_theme_listing,
+    websocket_error_message,
+)
 
 logger = logging.getLogger(__name__)
 
@@ -178,6 +182,7 @@ async def ha_manage_theme(
         ] = None,
         expected_current: Annotated[
             dict[str, Any] | None,
+            JSON_STRING_COERCION,
             Field(
                 description=(
                     "Compare-and-set guard for action='set_engine_theme': the "
@@ -191,6 +196,7 @@ async def ha_manage_theme(
         ] = None,
         value: Annotated[
             dict[str, Any] | None,
+            JSON_STRING_COERCION,
             Field(
                 description=(
                     "Frontend user-data theme object when "