[codex] Support assistant realtime append text

[guinness-oai]

Why

Frontend realtime voice continuity needs to replay a tiny previous-session overlap as actual conversation items, including assistant text. The app-server thread/realtime/appendText API already carries a role through to the Rust realtime websocket layer, but the shared role enum only accepted user and developer.

What Changed

• Added assistant to ConversationTextRole and regenerated the app-server schema/type fixtures.
• Added output_text as a realtime conversation content type.
• Updated realtime websocket item creation so assistant appendText emits content: [{ type: "output_text", text }], while user and developer continue to emit input_text.
• Updated app-server docs and tests to cover assistant appendText alongside the existing developer role behavior.

Validation

• just write-app-server-schema
• just fmt (first sandboxed attempt failed because uv could not access ~/.cache/uv; reran with filesystem access and passed)
• just test -p codex-api passed: 126/126
• just test -p codex-app-server-protocol passed: 239/239, including generated JSON/TypeScript fixture checks
• just test -p codex-app-server was started locally but stopped per request after unrelated local sandbox/Seatbelt failures (sandbox-exec: sandbox_apply: Operation not permitted) and one missing local codex binary failure; CI should be faster and more authoritative for the full suite.

[guinness-oai]

@codex

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

Reviewed commit: 943dffb813

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