fix: introduce AbsolutePathBuf as part of sandbox config (#7856)

Changes the `writable_roots` field of the `WorkspaceWrite` variant of
the `SandboxPolicy` enum from `Vec<PathBuf>` to `Vec<AbsolutePathBuf>`.
This is helpful because now callers can be sure the value is an absolute
path rather than a relative one. (Though when using an absolute path in
a Seatbelt config policy, we still have to _canonicalize_ it first.)

Because `writable_roots` can be read from a config file, it is important
that we are able to resolve relative paths properly using the parent
folder of the config file as the base path.

codex-rs/app-server-protocol/src/protocol/v1.rs

@@ -16,6 +16,7 @@ use codex_protocol::protocol::ReviewDecision;
 use codex_protocol::protocol::SandboxPolicy;
 use codex_protocol::protocol::SessionSource;
 use codex_protocol::protocol::TurnAbortReason;
+use codex_utils_absolute_path::AbsolutePathBuf;
 use schemars::JsonSchema;
 use serde::Deserialize;
 use serde::Serialize;
@@ -359,7 +360,7 @@ pub struct Tools {
 #[serde(rename_all = "camelCase")]
 pub struct SandboxSettings {
     #[serde(default)]
-    pub writable_roots: Vec<PathBuf>,
+    pub writable_roots: Vec<AbsolutePathBuf>,
     pub network_access: Option<bool>,
     pub exclude_tmpdir_env_var: Option<bool>,
     pub exclude_slash_tmp: Option<bool>,