feat: replace global gdb_controller with per-session GDB isolation

[Uday9909]

Problem

The backend currently uses a single global gdb_controller and program_name
shared across all requests:

gdb_controller = None  
program_name = None

If two users debug simultaneously, the second user's session silently
overwrites the first. This is the core blocker for multi-user support
and the sandbox architecture outlined in the project roadmap.

Expected Behavior

Each user should have an isolated GDB instance. Commands from one user
should never affect another user's debug session.