Reconfigure SecretsManagerClient after each code reload

What does this MR do and why?

SecretsManagement::SecretsManagerClient memoises its configuration on the class, and ee/config/initializers/secrets_manager.rb populated it once at boot. The class lives in ee/lib, which Zeitwerk reloads in development, so the first code change after boot replaced the class with one whose configuration had a nil host, and every OpenBao call raised ArgumentError: bad argument (expected URI object or URI string) until the server was restarted. Since Groups::SecretsController began logging into OpenBao during the page load, this surfaces as a 500 on the group Secrets page.

This MR runs the configuration inside Rails.application.config.to_prepare, which Rails executes again after every reload, matching how ee/config/initializers/policy_store.rb already handles the same problem. Production and test never reload, so nothing changes there. A spec proves the configuration is reapplied when the reloader's prepare callbacks run.

No changelog entry: the change only affects development mode.

References

• Resolves #622344

Differences

Before: In GDK, after any Rails code reload, visiting a group's Secrets page returns a 500 and every SecretsManagerClient call raises until restart.

After: The client is reconfigured on each reload and the page keeps working.

How to set up and validate locally

1. Start GDK with OpenBao and open a group's Settings > Secrets page.
2. Touch any file under ee/lib (or switch branches) and reload the page.
3. On master the page 500s with ArgumentError; with this MR it renders.

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