feat: add extension turn-input contributors

[jif-oai]

Disclaimer

Do not use for now

Why

Extensions can already contribute prompt fragments and request same-turn item injection, but there was no host-owned hook for contributing structured ResponseItems while Codex is assembling a new turn's initial model input. This change adds that seam so extensions can attach turn-local input that depends on the submitted user input and resolved turn environments without routing through prompt text or late injection.

What changed

• add TurnInputContributor to codex_extension_api and export the new TurnInputContext / TurnInputEnvironment types it receives
• teach ExtensionRegistry to register and expose turn-input contributors alongside the existing extension hooks
• call registered turn-input contributors from core/src/session/turn.rs while building the initial injected input for a turn, then append their returned ResponseItems after the skill and plugin injections

[chatgpt-codex-connector]

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