[FEATURE]: Implement V2 session.shell API #34428

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Implement the V2 session.shell behavior required by the TUI migration.

Current notes from #34359:

• SessionV2.shell exists as an interface method but currently returns OperationUnavailableError.
• The TUI needs a V2 session-level shell entrypoint rather than relying on the legacy SDK surface.
• This is related to, but separate from, V2: define shell backgrounding semantics #34366 shell backgrounding semantics.

Expected scope:

• Define the V2 session.shell input/output contract.
• Execute shell commands through the V2 session/tool/event flow.
• Surface useful shell start/progress/completion state to projected messages/events.
• Preserve permission checks and location/workdir behavior.
• Add protocol/server/client exposure if it belongs with the implementation.

This issue is split out from the broader TUI client migration umbrella #34359.