feat(sessions): add missing language, sortKey, and sortOrder filtering support

- Add language field with ISO 639-1 validation to sessionFilterSchema
- Add sortKey enum with startTime, category, language, sentiment, sessionId options
- Add sortOrder enum with asc/desc options
- Update tRPC router to support new filtering and sorting parameters
- Uncomment frontend code to enable full filtering functionality
- Add comprehensive validation tests for new schema fields

Resolves commented out filter options in app/dashboard/sessions/page.tsx lines 491-502

tests/unit/validation.test.ts

@@ -176,6 +176,36 @@ describe("Validation Schemas", () => {
       const data = { limit: 101 };
       expect(sessionFilterSchema.safeParse(data).success).toBe(false);
     });
+
+    it("should validate valid language code", () => {
+      const data = { language: "en" };
+      expect(sessionFilterSchema.safeParse(data).success).toBe(true);
+    });
+
+    it("should invalidate invalid language code", () => {
+      const data = { language: "invalid" };
+      expect(sessionFilterSchema.safeParse(data).success).toBe(false);
+    });
+
+    it("should validate valid sortKey", () => {
+      const data = { sortKey: "startTime" };
+      expect(sessionFilterSchema.safeParse(data).success).toBe(true);
+    });
+
+    it("should invalidate invalid sortKey", () => {
+      const data = { sortKey: "invalid" };
+      expect(sessionFilterSchema.safeParse(data).success).toBe(false);
+    });
+
+    it("should validate valid sortOrder", () => {
+      const data = { sortOrder: "asc" };
+      expect(sessionFilterSchema.safeParse(data).success).toBe(true);
+    });
+
+    it("should invalidate invalid sortOrder", () => {
+      const data = { sortOrder: "invalid" };
+      expect(sessionFilterSchema.safeParse(data).success).toBe(false);
+    });
   });
 
   describe("companySettingsSchema", () => {