Add lazy environment resolution for remote environments

[starr-openai]

Stacked on #20058.

Summary

• Move EnvironmentConfiguration from a stored websocket URL to a resolver-backed descriptor
• Add static and closure-backed resolver constructors for default and dynamic provider cases
• Keep EnvironmentManager as the id -> Environment cache while LazyRemoteExecServerClient resolves the URL on first use

Validation

• just fmt
• git diff --check
• bazel test --bes_backend= --bes_results_url= --test_output=errors //codex-rs/exec-server:all