Initialize execution environments with the final HTTP policy (#34995)

## Why

The TUI must inspect the default execution environment before loading its final
configuration. Initializing the environment manager at that point can give
startup services the bootstrap HTTP policy instead of the effective policy after
managed requirements are applied.

## What changed

- Split environment discovery from manager construction so callers can inspect
  the default environment without starting remote connections.
- Build the environment manager after final configuration loading and pass its
  resolved `HttpClientFactory` through all construction paths.
- Add shared test support for managers that use the legacy default HTTP policy.

## Testing

- Cover connection-free environment discovery and explicit HTTP policy
  propagation.
- Verify TUI startup services use the final managed `respect_system_proxy` value.

GitOrigin-RevId: 928fa31e6b4bcfbe1a121cade2f351427fdfa0f4

codex-rs/Cargo.toml

@@ -47,6 +47,7 @@ members = [
     "file-system",
     "exec-server-protocol",
     "exec-server",
+    "exec-server/tests/support",
     "execpolicy",
     "ext/agent",
     "ext/connectors",
@@ -181,6 +182,7 @@ codex-exec = { path = "exec" }
 codex-file-system = { path = "file-system" }
 codex-exec-server-protocol = { path = "exec-server-protocol" }
 codex-exec-server = { path = "exec-server" }
+codex-exec-server-test-support = { path = "exec-server/tests/support" }
 codex-execpolicy = { path = "execpolicy" }
 codex-extension-api = { path = "ext/extension-api" }
 codex-extension-items = { path = "ext/items" }