Keep shared MCP types stable in Code Mode exec descriptions (#50536)

## Why

Changing a deferred tool's output schema could add or remove the shared MCP type preamble from the `exec` description.

## What changed

Include the default or configured MCP TypeScript preamble whenever `code_mode_only` is enabled, regardless of the available tools' output schemas. Preserve an empty preamble override as a way to omit the section.

## Testing

Add a regression test that checks description stability when a deferred tool gains an MCP output schema, covering default, custom, and empty preambles. Update description expectations, request snapshots, and two Guardian context-budget test thresholds for the added preamble.

GitOrigin-RevId: 725035d6159e9da3c4beff5482c426c082e768b4

codex-rs/core/src/guardian/snapshots/codex_core__guardian__tests__guardian_followup_review_request_layout.snap

@@ -6,8 +6,8 @@ Scenario: Guardian follow-up review request layout
 
 ## Window 1
 -- request 1 (request; Initial Guardian Review Request) --
-00:additional_tools/developer (1; hash=ae2f2f1f9dcd0cd9):
-        - namespace/functions; hash=fd8638eaf7cabb95
+00:additional_tools/developer (1; hash=a8891bdc414756a2):
+        - namespace/functions; hash=af55b81b2cec6cc2
           - custom/exec
           - function/wait
 01:message/developer:

codex-rs/core/src/guardian/snapshots/codex_core__guardian__tests__guardian_review_request_layout.snap

@@ -6,8 +6,8 @@ Scenario: Guardian review request layout
 
 ## Window 1
 -- request 1 (request; Guardian Review Request) --
-00:additional_tools/developer (1; hash=ae2f2f1f9dcd0cd9):
-        - namespace/functions; hash=fd8638eaf7cabb95
+00:additional_tools/developer (1; hash=a8891bdc414756a2):
+        - namespace/functions; hash=af55b81b2cec6cc2
           - custom/exec
           - function/wait
 01:message/developer: