fix: prevent UTF-8 corruption when RTK truncation splits multi-byte codepoints (FAR-19)

The trunc function in the RTK filter script now walks back from the
truncation point past continuation bytes and checks whether the full
codepoint fits, avoiding replacement characters from mid-codepoint slicing.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/server/job-manifest.test.ts

@@ -802,6 +802,28 @@ describe("buildJobManifest", () => {
       expect(filterScript).toContain("tool_result");
     });
 
+    it("filter script truncates without corrupting multi-byte UTF-8", () => {
+      // "中" is U+4E2D, 3 bytes in UTF-8: E4 B8 AD
+      // With MAX=5, two "中" (6 bytes) should truncate to one (3 bytes), not
+      // produce a replacement character from slicing mid-codepoint.
+      const setup = buildRtkSetupCommands(5);
+      const b64Matches = [...setup.matchAll(/Buffer\.from\('([A-Za-z0-9+/=]+)','base64'\)/g)];
+      const filterScript = Buffer.from(b64Matches[0]![1], "base64").toString("utf-8");
+
+      // Extract the trunc function from the filter script and evaluate it
+      const fnMatch = filterScript.match(/(function trunc\(s\)\{.*\})(?=const tr=)/);
+      expect(fnMatch).toBeTruthy();
+      // eslint-disable-next-line no-eval
+      const trunc = eval(`(()=>{const MAX=5;${fnMatch![1]};return trunc;})()`);
+
+      const result = trunc("中中");
+      expect(result).not.toContain("�");
+      expect(result).toContain("中");
+      expect(result).toContain("truncated by paperclip-rtk");
+      // Should report bytes from the actual truncation point, not MAX
+      expect(result).toContain("3 bytes truncated");
+    });
+
     it("filter script handles array content (block format)", () => {
       const setup = buildRtkSetupCommands(50000);
       const b64Matches = [...setup.matchAll(/Buffer\.from\('([A-Za-z0-9+/=]+)','base64'\)/g)];

src/server/job-manifest.ts

@@ -47,7 +47,8 @@ export function buildRtkSetupCommands(maxOutputBytes: number): string {
     `if(typeof s!=='string')return s;`,
     `const b=Buffer.from(s,'utf-8');`,
     `if(b.length<=MAX)return s;`,
-    `return b.slice(0,MAX).toString('utf-8')+'\\n[...'+(b.length-MAX)+' bytes truncated by paperclip-rtk]';`,
+    `let e=MAX;if(e>0){let p=e-1;while(p>0&&(b[p]&0xC0)===0x80)p--;const l=b[p];let n=1;if((l&0xE0)===0xC0)n=2;else if((l&0xF0)===0xE0)n=3;else if((l&0xF8)===0xF0)n=4;if(p+n>e)e=p;}`,
+    `return b.slice(0,e).toString('utf-8')+'\\n[...'+(b.length-e)+' bytes truncated by paperclip-rtk]';`,
     `}`,
     `const tr=o&&(o.tool_response||o.tool_result);`,
     `if(tr){`,