Add environment provider snapshot (#20058)

## Summary
- Change `EnvironmentProvider` to return concrete `Environment`
instances instead of `EnvironmentConfigurations`.
- Make `DefaultEnvironmentProvider` provide the provider-visible `local`
environment plus optional `remote` environment from
`CODEX_EXEC_SERVER_URL`.
- Keep `EnvironmentManager` as the concrete cache while exposing its own
explicit local environment for `local_environment()` fallback paths.

## Validation
- `just fmt`
- `git diff --check`

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

codex-rs/exec-server/src/lib.rs

@@ -2,6 +2,7 @@ mod client;
 mod client_api;
 mod connection;
 mod environment;
+mod environment_provider;
 mod fs_helper;
 mod fs_helper_main;
 mod fs_sandbox;
@@ -38,6 +39,8 @@ pub use environment::EnvironmentManager;
 pub use environment::EnvironmentManagerArgs;
 pub use environment::LOCAL_ENVIRONMENT_ID;
 pub use environment::REMOTE_ENVIRONMENT_ID;
+pub use environment_provider::DefaultEnvironmentProvider;
+pub use environment_provider::EnvironmentProvider;
 pub use fs_helper::CODEX_FS_HELPER_ARG1;
 pub use fs_helper_main::main as run_fs_helper_main;
 pub use local_file_system::LOCAL_FS;