fix(agents): use alphanumeric-only tool call IDs for OpenRouter compatibility

Some providers like Mistral via OpenRouter require strictly alphanumeric
tool call IDs. The error message indicates: "Tool call id was
whatsapp_login_1768799841527_1 but must be a-z, A-Z, 0-9, with a length
of 9."

Changes:
- Update sanitizeToolCallId to strip all non-alphanumeric characters
  (previously allowed underscores and hyphens)
- Update makeUniqueToolId to use alphanumeric suffixes (x2, x3, etc.)
  instead of underscores
- Update isValidCloudCodeAssistToolId to validate alphanumeric-only IDs
- Update tests to reflect stricter sanitization

Fixes #1359

Co-Authored-By: Claude <noreply@anthropic.com>

src/agents/pi-embedded-helpers.sanitizetoolcallid.test.ts

@@ -10,13 +10,15 @@ const _makeFile = (overrides: Partial<WorkspaceBootstrapFile>): WorkspaceBootstr
   ...overrides,
 });
 describe("sanitizeToolCallId", () => {
-  it("keeps valid tool call IDs", () => {
-    expect(sanitizeToolCallId("call_abc-123")).toBe("call_abc-123");
+  it("keeps valid alphanumeric tool call IDs", () => {
+    expect(sanitizeToolCallId("callabc123")).toBe("callabc123");
   });
-  it("replaces invalid characters with underscores", () => {
-    expect(sanitizeToolCallId("call_abc|item:456")).toBe("call_abc_item_456");
+  it("strips non-alphanumeric characters (Mistral/OpenRouter compatibility)", () => {
+    expect(sanitizeToolCallId("call_abc-123")).toBe("callabc123");
+    expect(sanitizeToolCallId("call_abc|item:456")).toBe("callabcitem456");
+    expect(sanitizeToolCallId("whatsapp_login_1768799841527_1")).toBe("whatsapplogin17687998415271");
   });
   it("returns default for empty IDs", () => {
-    expect(sanitizeToolCallId("")).toBe("default_tool_id");
+    expect(sanitizeToolCallId("")).toBe("defaulttoolid");
   });
 });