[codex] Fix realtime v1 websocket compatibility

[guinness-oai]

Why

Realtime v1 websocket sessions now expect a slightly different boundary shape for text input, completed input transcripts, and connection headers. Codex was still using the older shape, so some v1 text appends could be rejected before the existing conversation flow could handle them.

What changed

• Send v1 user text items with input_text content
• Accept v1 turn-marked input transcript events as completed transcripts
• Add the v1 alpha header only for v1 realtime sessions
• Cover the outbound text shape, transcript parsing, and versioned headers

Test plan

• cargo test -p codex-api endpoint::realtime_websocket::methods::tests
• cargo test -p codex-core quicksilver_alpha_header