Consider a lightweight progress content block for streaming non-terminal-backed shell output

Follow-up from #314, discussion thread: https://github.com/microsoft/agent-host-protocol/pull/314#discussion_r3540133509

Rob asked whether shell exit content should be separated so terminal output can stream. We resolved this for the case where a full ahp-terminal: channel backs the command: a host emits a live ToolResultTerminalContent block while it runs, then a ToolResultTerminalCompleteContent block referencing the same resource when it exits.

That doesn't cover the lighter case — a plain shell child process with no subscribable terminal channel. There's currently no way for a host to stream partial output/progress for that kind of execution before the final TerminalComplete summary lands.

Problem

• ToolResultTerminalContent implies a full terminal channel (cols/rows, command detection, etc.) — heavier than some shell tools want just to report incremental output.
• Non-terminal-backed shells only get one shot: the final preview in TerminalComplete, after the command has already exited.

Proposal (not fully speced yet)

• A lighter content type for incremental output from a running command that isn't backed by a full terminal channel. Naming isn't settled — ToolResultShellProgressContent vs ToolResultTerminalProgressContent vs something else — depends on whether we want this to read as shell-specific or as part of the same terminal-content family as ToolResultTerminalContent/ToolResultTerminalCompleteContent.
• Needs to stay clearly distinct from ToolResultTerminalCompleteContent, which is specifically the post-exit record.

Sequencing

We should land #322 (PTY / non-PTY capability detection) before this. Once clients can tell whether an execution is PTY-backed vs a plain child process, that same signal likely informs whether/how a progress block should behave — don't want to design streaming semantics twice.

Related

• #314 — where this was raised and partially resolved (terminal-backed case)
• #322 — capability detection for PTY vs non-PTY terminal execution (should land first)