core: wrap token budget window context (#29494)

Token-budget initial context carries thread and context-window lineage
that the model should treat as one structured context-window block.
Wrapping it in `<context_window>` makes that boundary explicit while
preserving the existing window id content.

Before this change, the window identifiers were injected as an untagged
developer text fragment:

```text
Thread id <THREAD_ID>.
First context window id: <FIRST_WINDOW_ID>
Current context window id: <WINDOW_ID>
Previous context window id: <PREVIOUS_WINDOW_ID>
```

After this change, the same payload is wrapped as a context-window
block:

```text
<context_window>
Thread id: <THREAD_ID>
First context window id: <FIRST_WINDOW_ID>
Current context window id: <WINDOW_ID>
Previous context window id: <PREVIOUS_WINDOW_ID>
</context_window>
```

This adds shared `CONTEXT_WINDOW_*_TAG` protocol constants, updates
`TokenBudgetContext` to render with those markers, treats the new
wrapper as contextual developer content when mapping history, and
refreshes the token-budget request-shape assertions and snapshot.

Verification:
- `just test -p codex-core token_budget`
- `just test -p codex-core
recognizes_context_window_as_contextual_developer_content`

codex-rs/protocol/src/protocol.rs

@@ -92,8 +92,8 @@ use crate::permissions::default_read_only_subpaths_for_writable_root;
 pub use crate::request_permissions::RequestPermissionsArgs;
 pub use crate::request_user_input::RequestUserInputEvent;
 
-/// Open/close tags for special user-input blocks. Used across crates to avoid
-/// duplicated hardcoded strings.
+/// Open/close tags for special context blocks. Used across crates to avoid duplicated hardcoded
+/// strings.
 pub const USER_INSTRUCTIONS_OPEN_TAG: &str = "<user_instructions>";
 pub const USER_INSTRUCTIONS_CLOSE_TAG: &str = "</user_instructions>";
 pub const ENVIRONMENT_CONTEXT_OPEN_TAG: &str = "<environment_context>";
@@ -110,6 +110,8 @@ pub const MULTI_AGENT_MODE_OPEN_TAG: &str = "<multi_agent_mode>";
 pub const MULTI_AGENT_MODE_CLOSE_TAG: &str = "</multi_agent_mode>";
 pub const REALTIME_CONVERSATION_OPEN_TAG: &str = "<realtime_conversation>";
 pub const REALTIME_CONVERSATION_CLOSE_TAG: &str = "</realtime_conversation>";
+pub const CONTEXT_WINDOW_OPEN_TAG: &str = "<context_window>";
+pub const CONTEXT_WINDOW_CLOSE_TAG: &str = "</context_window>";
 pub const USER_MESSAGE_BEGIN: &str = "## My request for Codex:";
 
 // TODO(anp): Replace `TurnEnvironmentSelection` with `PathUri` once path URIs carry environment