fix(typecheck): add missing xai entry to deprecation retirement date maps (#1509)

The LegacyAPIProvider union type includes 'xai' but the three
DEPRECATED_MODELS entries were missing the corresponding key,
causing TS2741 'Property xai is missing' errors.

Refs: #1486

src/utils/model/deprecation.ts

@@ -46,6 +46,7 @@ const DEPRECATED_MODELS: Record<string, DeprecationEntry> = {
       minimax: null,
       'xiaomi-mimo': null,
       mistral: null,
+      xai: null,
     },
   },
   'claude-3-7-sonnet': {
@@ -63,6 +64,7 @@ const DEPRECATED_MODELS: Record<string, DeprecationEntry> = {
       minimax: null,
       'xiaomi-mimo': null,
       mistral: null,
+      xai: null,
     },
   },
   'claude-3-5-haiku': {
@@ -80,6 +82,7 @@ const DEPRECATED_MODELS: Record<string, DeprecationEntry> = {
       minimax: null,
       'xiaomi-mimo': null,
       mistral: null,
+      xai: null,
     },
   },
 }