Make environment provider snapshots path-free

[starr-openai]

Summary

• make EnvironmentProvider::snapshot path-free and keep providers focused on provider-owned remote environments
• let provider snapshots request local inclusion via include_local, with environments.toml including local and CODEX_EXEC_SERVER_URL excluding local
• move reserved local environment construction into EnvironmentManager using ExecServerRuntimePaths

Follow-up to #20667

Testing

• just fmt
• git diff --check
• devbox: bazel build --bes_backend= --bes_results_url= //codex-rs/exec-server:exec-server
• devbox: bazel test --bes_backend= --bes_results_url= //codex-rs/exec-server:exec-server-unit-tests

[chatgpt-codex-connector]

💡 Codex Review

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

Line 669 in 2ae2034

assert!(!Arc::ptr_eq(&provider_local, &manager.local_environment()));

Update stale local environment pointer assertion

from_provider_snapshot now inserts Arc::clone(&local_environment) for the reserved local id, so get_environment("local") and local_environment() intentionally point to the same Arc. This unchanged negative assertion will panic and fail the exec-server test suite whenever this test runs.

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