Adds WebRTC startup to the experimental app-server
`thread/realtime/start` method with an optional transport enum. The
websocket path remains the default; WebRTC offers create the realtime
session through the shared start flow and emit the answer SDP via
`thread/realtime/sdp`.
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Co-authored-by: Codex <noreply@openai.com>
## Stack Position
2/4. Built on top of #14828.
## Base
- #14828
## Unblocks
- #14829
- #14827
## Scope
- Port the realtime v2 wire parsing, session, app-server, and
conversation runtime behavior onto the split websocket-method base.
- Branch runtime behavior directly on the current realtime session kind
instead of parser-derived flow flags.
- Keep regression coverage in the existing e2e suites.
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Co-authored-by: Codex <noreply@openai.com>
## Stack Position
1/4. Base PR in the realtime stack.
## Base
- `main`
## Unblocks
- #14830
## Scope
- Split the realtime websocket request builders into `common`, `v1`, and
`v2` modules.
- Keep runtime behavior unchanged in this PR.
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Co-authored-by: Codex <noreply@openai.com>
