[codex] Add optional IDs to response items

[pakrym-oai]

Why

ResponseItem variants do not have a consistent internal ID shape: some variants carry required IDs, some carry optional IDs, and some cannot represent an ID at all. The existing fields also use inconsistent serde, TypeScript, and JSON-schema annotations. A single enum-level access path is needed before history recording can assign and retain IDs.

This PR establishes that internal model only. It intentionally does not generate or serialize IDs; allocation and wire persistence are isolated in the stacked follow-up.

What changed

• Give every concrete ResponseItem variant an Option<String> ID field.
• Apply the same internal-only annotations to every ID field: #[serde(default, skip_serializing)], #[ts(skip)], and #[schemars(skip)].
• Add ResponseItem::id() and ResponseItem::set_id() as the shared accessors.
• Preserve IDs when history items are rewritten for truncation.
• Adapt consumers that previously assumed reasoning and image-generation IDs were required.
• Regenerate app-server schemas so the hidden fields are represented consistently.

The serde catch-all ResponseItem::Other remains ID-less because it must remain a unit variant.

Test plan

• cargo check --tests -p codex-core -p codex-api -p codex-rollout-trace -p codex-image-generation-extension
• just test -p codex-protocol
• just test -p codex-app-server-protocol
• just test -p codex-api -p codex-rollout-trace -p codex-image-generation-extension
• just test -p codex-core event_mapping

[chatgpt-codex-connector]

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