Route process tools to selected environments (#20647)

## Why
When a turn exposes multiple selected environments, shell-style tools
need a model-facing way to identify the intended target environment and
handlers need to resolve that target before parsing cwd-relative
permission fields or launching processes.

This PR scopes that rollout to process tools. Filesystem-oriented tools
such as `apply_patch`, `view_image`, and `list_dir` are intentionally
left for follow-up slices.

## What Changed
- Adds an `include_environment_id` option to shell-style tool schema
builders.
- Exposes optional `environment_id` on `shell`, `shell_command`, and
`exec_command` only when `ToolEnvironmentMode::Multiple` is active.
- Adds a shared handler helper that parses `environment_id` and
`workdir` from JSON function-call arguments and returns the selected
`Environment` plus effective absolute cwd.
- Uses that helper in `shell`, `shell_command`, and `exec_command`
handling so process execution uses the selected environment filesystem
and cwd.
- Changes `ExecCommandRequest` to carry a required resolved `cwd`,
removing the process-manager fallback to the primary turn cwd for new
exec commands.
- Leaves `write_stdin` unchanged because it targets an existing process
id, not a new environment.

## Testing
- Added unit coverage for process-tool schema exposure, selected
environment resolution, primary fallback, no-environment handling,
unknown environment ids, and resolving cwd-relative permission paths
against the selected environment cwd.
- Added a remote-suite e2e coverage case for `exec_command` routing
across explicit zero environments, one local environment, and
local+remote environments.
- Ran `just fmt` and `git diff --check`.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/unified_exec/mod.rs

@@ -27,6 +27,7 @@ use std::collections::HashSet;
 use std::sync::Arc;
 use std::sync::Weak;
 
+use codex_exec_server::Environment;
 use codex_network_proxy::NetworkProxy;
 use codex_protocol::models::AdditionalPermissionProfile;
 use codex_utils_absolute_path::AbsolutePathBuf;
@@ -93,7 +94,8 @@ pub(crate) struct ExecCommandRequest {
     pub process_id: i32,
     pub yield_time_ms: u64,
     pub max_output_tokens: Option<usize>,
-    pub workdir: Option<AbsolutePathBuf>,
+    pub cwd: AbsolutePathBuf,
+    pub environment: Arc<Environment>,
     pub network: Option<NetworkProxy>,
     pub tty: bool,
     pub sandbox_permissions: SandboxPermissions,