🤖 fix: add contextUsage fields to MuxMessageSchema

The MuxMessageSchema metadata object was missing contextUsage and
contextProviderMetadata fields. When messages were serialized through
oRPC/Zod, these fields were silently stripped (Zod's default behavior).

This caused context usage to display cumulative usage (from the 'usage'
field) instead of last-step usage (from 'contextUsage'), resulting in
wildly inflated percentages (e.g., 944% instead of ~50%) after app restart.

The bug only appeared after restart because during live streaming, the
frontend received the fields before oRPC serialization. On reload, messages
pass through the schema and lose the fields.

_Generated with mux_

Author: ammar-agent

src/common/orpc/schemas/message.ts

@@ -76,7 +76,11 @@ export const MuxMessageSchema = z.object({
       timestamp: z.number().optional(),
       model: z.string().optional(),
       usage: z.any().optional(),
+      // Last step's usage only (for context window display)
+      contextUsage: z.any().optional(),
       providerMetadata: z.record(z.string(), z.unknown()).optional(),
+      // Last step's provider metadata (for context window cache display)
+      contextProviderMetadata: z.record(z.string(), z.unknown()).optional(),
       duration: z.number().optional(),
       systemMessageTokens: z.number().optional(),
       muxMetadata: z.any().optional(),