fix: F5 debug launch not entering interactive mode

[ayeshurun]

Problem

Pressing F5 and submitting an empty argument prompt printed the welcome banner and then exited with a bare Error in interactive session: instead of entering the REPL.

Reproduced locally. Two independent defects, both pre-existing (.vscode/launch.json is unchanged since the initial commit ac4dc95).

Root cause

1. .vscode/launch.json

• The default configuration used "args": "${command:pickArgs}". An empty prompt yields [], so main.py correctly falls through to interactive mode — the prompt was just noise on the common path.
• The second configuration used "console": "externalTerminal" with stopOnEntry, which cannot attach a terminal inside a dev container.
• Both used the deprecated ${workspaceRoot} instead of ${workspaceFolder}.

2. src/fabric_cli/core/fab_interactive.py

EOFError is a subclass of Exception, so the loop's generic except Exception as e caught it before the outer except (EOFError, KeyboardInterrupt) — making that outer handler unreachable from inside the loop. Since str(EOFError()) is the empty string, the message printed was Error in interactive session: with nothing after it.

This affected two cases:

• Ctrl+D on a normal terminal (should print the goodbye message).
• Non-terminal stdin, where prompt_toolkit raises EOFError immediately at the first prompt.

Changes

.vscode/launch.json

• Default config Fabric CLI Debug (Interactive): "args": [], "module": "fabric_cli.main", "console": "integratedTerminal". Interactive mode requires a TTY, which the Debug Console does not provide.
• Second config Fabric CLI Debug (Prompt for args) keeps ${command:pickArgs} for one-shot commands, also on the integrated terminal.
• Removed the externalTerminal / stopOnEntry configuration.
• ${workspaceRoot} → ${workspaceFolder}.

src/fabric_cli/core/fab_interactive.py

• Re-raise EOFError from the loop so the outer handler runs.
• Split the outer handler into except EOFError and except KeyboardInterrupt. On EOFError, if stdin is not a TTY, additionally explain why interactive mode cannot start; the normal exit message is printed in both branches.

tests/test_core/test_fab_interactive.py

test_start_interactive_eof_error_failure asserted the buggy blank message, so it encoded the defect and had to be rewritten. Added coverage for terminal and non-terminal stdin. interactive_cli.session.prompt is a module-level mock shared through the InteractiveCLI singleton, so reset_mock() is required before asserting call_count.

Verification

• Real TTY (via pty.fork): banner → fab:/$ prompt → exit → goodbye message.
• Non-TTY stdin: banner → explanation → goodbye message, exit code 0. Previously: blank error.
• python3 -m pytest tests/test_core tests/test_utils → 553 passed.
• mypy clean on both changed files; black --check reports no new diffs (3 pre-existing violations in these files are left untouched).

Not verified

I could not determine why the reporter's debug session had non-terminal stdin — that requires VS Code and debugpy, which are not available in this environment. The fix handles every branch regardless, and the non-TTY case now reports its own cause rather than failing silently.