repl: Improve kernelspec discoverability (#15886)

<img width="862" alt="image"
src="https://github.com/user-attachments/assets/ae8c479d-d9f9-4c46-bb1a-be411ab07876">

Release Notes:

- Added additional context about available to kernel sessions
- Fixed bug in kernelspec launch choosing first available kernel
matching the language rather than selected name

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Co-authored-by: Jason <jason@zed.dev>