feat(lsp): add debugOverrideCommand option

[shorwood]

Closes #22128

Adds debugOverrideCommand for test and bench runnables so debug can use a different command from Run. This is needed for runners like nextest, where running a test and debugging a test require different command shapes.

The setting is opt-in. Existing overrideCommand, Cargo debug config generation, and matching launch.json entries keep their current behavior.

For reference, here's my setup with this PR active.

{
    "rust-analyzer.server.path": "$HOME/Workspaces/rust-analyzer/target/debug/rust-analyzer",
    "rust-analyzer.runnables.test.overrideCommand": [
        "nix",
        "shell",
        "nixpkgs#cargo-nextest",
        "-c",
        "cargo",
        "nextest",
        "run",
        "--config",
        "profile.dev.debug=2",
        "--package",
        "${package}",
        "${target_arg}",
        "${target}",
        "--",
        "${test_name}",
        "${exact}",
        "${include_ignored}"
    ],
    "rust-analyzer.runnables.test.debugOverrideCommand": [
        "nix",
        "shell",
        "nixpkgs#cargo-nextest",
        "-c",
        "cargo",
        "nextest",
        "run",
        "--config",
        "profile.dev.debug=2",
        "--package",
        "${package}",
        "${target_arg}",
        "${target}",
        "--debugger",
        "$HOME/.vscode/extensions/vadimcn.vscode-lldb-1.12.1/bin/codelldb-launch --connect 127.0.0.1:12345 --",
        "--",
        "${test_name}",
        "${exact}",
        "${include_ignored}"
    ],
    "rust-analyzer.runnables.extraEnv": {
        "CODELLDB_LAUNCH_CONFIG": "{ token: 'secret' }"
    },
    "lldb.rpcServer": {
        "host": "127.0.0.1",
        "port": 12345,
        "token": "secret"
    }
}

[rustbot]

☔ The latest upstream changes (possibly #22284) made this pull request unmergeable. Please resolve the merge conflicts.