Aspire CLI: allow overriding terminal attach leader key (Ctrl+B)

[mitchdenny]

Feature request

aspire terminal attach currently uses Ctrl+B as the leader key for CLI-level controls. This conflicts with TUIs that already bind Ctrl+B, making those interactions hard or impossible to test through Aspire terminal attach.

Problem

When the attached app/TUI expects Ctrl+B, the Aspire attach layer intercepts it as the leader key.

Proposal

Add a CLI option to override the leader key, for example:

aspire terminal attach <resource> --leader-key CTRL-L

Goals

• Allow users to choose a non-conflicting leader key per attach session.
• Keep a default leader key for current behavior.
• Clearly document accepted key formats and invalid combinations.

Why this matters

This unblocks testing and interactive use of TUIs that rely on Ctrl+B bindings while still preserving Aspire attach controls.