fix: default unknown tool schemas to empty schemas (#22380)

## Why

Some tool providers, especially MCP servers and dynamic tool sources,
can supply schema nodes that omit `type` and have no recognized JSON
Schema shape hints. Previously, `sanitize_json_schema` filled those
unknown nodes in as `string`, which made the schema parseable but
invented a scalar constraint that the provider did not specify. For
description-only fields, that could incorrectly steer tool arguments
away from the provider's actual accepted shape.

The Responses API accepts permissive empty schemas such as `{}` at
nested property positions, so Codex should preserve that permissive
meaning instead of coercing unknown schema nodes into a misleading
scalar type.

## What Changed

- Changed the no-hints fallback in `codex-rs/tools/src/json_schema.rs`
to clear unrecognized object schema nodes to `{}`.
- Empty schemas now remain `{}` rather than becoming `type: "string"`.
- Description-only or otherwise metadata-only nested property schemas
now become `{}` while surrounding object/array/string/number inference
still applies when recognized hints are present.
- Updated `codex-tools` and `codex-core` tests to cover top-level empty
schemas, nested empty schemas, metadata-only malformed schemas, dynamic
tools, and MCP tool specs.

## Verification

- `cargo test -p codex-tools`
- `cargo test -p codex-core
test_mcp_tool_property_missing_type_defaults_to_empty_schema`
- Manually verified the real Responses API behavior for both
empty-schema positions:
- Top-level function `parameters: {}` is accepted and echoed back as
`{"type":"object","properties":{}}`; when forced to call the tool,
Responses emitted empty object arguments: `"arguments": "{}"`.
- Nested property schema `{}` is accepted and preserved as `{}`; when
forced to call a tool with `metadata.extra`, Responses emitted
`"arguments": "{\"metadata\":{\"extra\":\"codex schema sanitizer
behavior\"}}"`.

codex-rs/tools/src/json_schema.rs

@@ -166,6 +166,7 @@ pub fn parse_tool_input_schema(input_schema: &JsonValue) -> Result<JsonSchema, s
 /// - Collapses `const` into single-value `enum`.
 /// - Fills required child fields for object/array schema types, including
 ///   nullable unions, with permissive defaults when absent.
+/// - Coerces object schemas with no recognized schema hints into `{}`.
 fn sanitize_json_schema(value: &mut JsonValue) {
     match value {
         JsonValue::Bool(_) => {
@@ -228,7 +229,8 @@ fn sanitize_json_schema(value: &mut JsonValue) {
                 {
                     schema_types.push(JsonSchemaPrimitiveType::Number);
                 } else {
-                    schema_types.push(JsonSchemaPrimitiveType::String);
+                    map.clear();
+                    return;
                 }
             }