feat(core): add structured network approval plumbing and policy decision model (#11672)

### Description
#### Summary
Introduces the core plumbing required for structured network approvals

#### What changed
- Added structured network policy decision modeling in core.
- Added approval payload/context types needed for network approval
semantics.
- Wired shell/unified-exec runtime plumbing to consume structured
decisions.
- Updated related core error/event surfaces for structured handling.
- Updated protocol plumbing used by core approval flow.
- Included small CLI debug sandbox compatibility updates needed by this
layer.

#### Why
establishes the minimal backend foundation for network approvals without
yet changing high-level orchestration or TUI behavior.

#### Notes
- Behavior remains constrained by existing requirements/config gating.
- Follow-up PRs in the stack handle orchestration, UX, and app-server
integration.

---------

Co-authored-by: Codex <199175422+chatgpt-codex-connector[bot]@users.noreply.github.com>

codex-rs/protocol/src/protocol.rs

@@ -54,6 +54,8 @@ pub use crate::approvals::ApplyPatchApprovalRequestEvent;
 pub use crate::approvals::ElicitationAction;
 pub use crate::approvals::ExecApprovalRequestEvent;
 pub use crate::approvals::ExecPolicyAmendment;
+pub use crate::approvals::NetworkApprovalContext;
+pub use crate::approvals::NetworkApprovalProtocol;
 pub use crate::request_user_input::RequestUserInputEvent;
 
 /// Open/close tags for special user-input blocks. Used across crates to avoid