memories/mcp: generate tool schemas with schemars (#21012)

## Why

The memories MCP server currently keeps handwritten JSON Schema beside
the Rust types that actually serialize and deserialize the tool
payloads:
[`schema.rs`](https://github.com/openai/codex/blob/2f5c06a29cdd68f11d07126dc56871bff1218ba1/codex-rs/memories/mcp/src/schema.rs#L4-L133),
[`server.rs`](https://github.com/openai/codex/blob/2f5c06a29cdd68f11d07126dc56871bff1218ba1/codex-rs/memories/mcp/src/server.rs#L44-L75),
and
[`backend.rs`](https://github.com/openai/codex/blob/2f5c06a29cdd68f11d07126dc56871bff1218ba1/codex-rs/memories/mcp/src/backend.rs#L41-L117).
That duplicates the tool contract and makes schema drift easier as the
API evolves.

## What changed

- derive `JsonSchema` for the memories tool arguments, responses, and
nested response types
- replace the handwritten schema builders with shared `schemars`
generation
- preserve the existing wire shape while generating schemas, including
nullable output `Option` fields and non-nullable optional input fields
- wire the `list`, `read`, and `search` tools to the generated schemas

## Verification

- CI pending

codex-rs/Cargo.lock

@@ -3047,6 +3047,7 @@ dependencies = [
  "codex-utils-output-truncation",
  "pretty_assertions",
  "rmcp",
+ "schemars 0.8.22",
  "serde",
  "serde_json",
  "tempfile",