Add supports_parallel_tool_calls flag to included mcps (#17667)

## Why

For more advanced MCP usage, we want the model to be able to emit
parallel MCP tool calls and have Codex execute eligible ones
concurrently, instead of forcing all MCP calls through the serial block.

The main design choice was where to thread the config. I made this
server-level because parallel safety depends on the MCP server
implementation. Codex reads the flag from `mcp_servers`, threads the
opted-in server names into `ToolRouter`, and checks the parsed
`ToolPayload::Mcp { server, .. }` at execution time. That avoids relying
on model-visible tool names, which can be incomplete in
deferred/search-tool paths or ambiguous for similarly named
servers/tools.

## What was added

Added `supports_parallel_tool_calls` for MCP servers.

Before:

```toml
[mcp_servers.docs]
command = "docs-server"
```

After:

```toml
[mcp_servers.docs]
command = "docs-server"
supports_parallel_tool_calls = true
```

MCP calls remain serial by default. Only tools from opted-in servers are
eligible to run in parallel. Docs also now warn to enable this only when
the server’s tools are safe to run concurrently, especially around
shared state or read/write races.

## Testing

Tested with a local stdio MCP server exposing real delay tools. The
model/Responses side was mocked only to deterministically emit two MCP
calls in the same turn.

Each test called `query_with_delay` and `query_with_delay_2` with `{
"seconds": 25 }`.

| Build/config | Observed | Wall time |
| --- | --- | --- |
| main with flag enabled | serial | `58.79s` |
| PR with flag enabled | parallel | `31.73s` |
| PR without flag | serial | `56.70s` |

PR with flag enabled showed both tools start before either completed;
main and PR-without-flag completed the first delay before starting the
second.

Also added an integration test.

Additional checks:

- `cargo test -p codex-tools` passed
- `cargo test -p codex-core
mcp_parallel_support_uses_exact_payload_server` passed
- `git diff --check` passed

codex-rs/core/tests/suite/sqlite_state.rs

@@ -368,6 +368,7 @@ async fn mcp_call_marks_thread_memory_mode_polluted_when_configured() -> Result<
                 },
                 enabled: true,
                 required: false,
+                supports_parallel_tool_calls: false,
                 disabled_reason: None,
                 startup_timeout_sec: Some(Duration::from_secs(10)),
                 tool_timeout_sec: None,