feat: add config-change extension contributor

[jif-oai]

Why

Extensions can observe thread and turn lifecycle events today, but there was no single host-owned hook for changes to the effective thread configuration. That makes features that need to react to model, permission, or tool-suggest updates either depend on individual mutation paths or risk going stale after runtime config refreshes.

This adds a typed config-change contributor so extension-owned state can stay synchronized with the effective thread config while the host remains responsible for deciding when config changed.

What Changed

• Added ConfigContributor<C> to codex_extension_api, with before/after immutable snapshots of the effective config plus session/thread extension stores.
• Added registry builder/accessor support through config_contributor and config_contributors.
• Emits config-change callbacks after committed updates from session settings, per-turn setting updates, and refresh_runtime_config.
• Builds effective config snapshots only when config contributors are registered, and suppresses no-op callbacks when the before/after snapshots are equal.
• Added a core session regression test that verifies contributors observe both model changes and user-layer runtime config changes, including access to session and thread extension stores.

Validation

Added config_change_contributor_observes_effective_config_changes in codex-rs/core/src/session/tests.rs to cover the new contributor path.

[jif-oai]

@codex review

[chatgpt-codex-connector]

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