exec-server: support pending environment readiness

[sayan-oai]

Why

CCA environments only have a stable exec-server URL once the filesystem and execution environment are genuinely ready. Requiring that URL during registration forces those providers either to delay registration or to expose an endpoint before the environment can actually be used.

This change lets an in-process provider register the environment ID immediately and publish the stable URL at the provider's canonical readiness transition. Codex then follows the existing WebSocket connection and initialization path, so downstream environment selection and wait_for_environment behavior continue to use the established readiness contract.

What changed

• add EnvironmentManager methods to register a pending environment, publish its exec-server URL, or fail and remove it
• represent ready and deferred URLs with one shared, one-shot ExecServerUrl source used by the existing WebSocket transport
• reject duplicate pending registrations and fail existing waiters when a pending environment is replaced
• preserve the existing direct-URL registration and reconnect behavior
• exercise manager-driven starting to ready and starting to unavailable transitions through the core deferred-environment flow

[sayan-oai]

@codex review this

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

Reviewed commit: 358eb9db23

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