Fix dap request serialization and potential gutter underflow

[Mr-Manatee]

• Fix underflow in breakpoint or diagnostic gutter fn
• Fix helix-dap threads request arguments serialization

I ran into issues while debugging in helix.

I noticed that #14879 resolved a similar issue and applied it to the issue that I was having according to the DAP logs.

After resolving that issue I uncovered a potential underflow when the dap response body references line 0. I encountered this when a break point encountered a libc function.

2025-12-24T22:23:53.268 helix_dap::transport [INFO] [1v1] <- DAP {"request_seq": 51, "type": "response", "command": "stackTrace", "success": true, "body": {"stackFrames": [{"id": 9, "line": 0, "column": 0, "instructionPointerReference": "0x7ffff7cd4172", "moduleId": "/usr/lib64/libc.so.6", "name": "???"}, ... yada yada...