code-mode: make cell creation idempotent

[cconger]

Why

A create response can be canceled or lost after SessionRuntime admits a cell. Retrying without stable request identity can start a second cell while leaving the first undiscoverable.

What

• Add an idempotency key to runtime cell creation.
• Linearize concurrent creates for the same key with a shared initialization cell.
• Return the same cell ID for a repeated matching create.
• Reject reuse of a key across continuing and pausable cell kinds.
• Add repeated and concurrent creation coverage.

Stack boundary

The key exists only on the transport-neutral runtime request in this PR. The session protocol still derives runtime identity internally; #29397 carries a caller-provided key across the host/core boundary. Core-generated cell IDs replace the separate key later in #29398.

Validation

• just test -p codex-code-mode
• Runtime regressions cover repeated and concurrent idempotent creation.

Stack parent: #29400.