fix: remove invalid prompt_toolkit ctrl-shift-c binding

[emma17bot]

Summary

• Remove the invalid c-S-c prompt_toolkit binding that prevents the CLI from starting
• Let terminal emulators continue handling Ctrl+Shift+C copy natively
• Add a regression test validating hard-coded CLI keybindings against prompt_toolkit

Test Plan

• python -m pytest tests/hermes_cli/test_cli_keybindings.py -q
• python -m py_compile cli.py tests/hermes_cli/test_cli_keybindings.py
• timeout 5s hermes startup smoke test

[alt-glitch]

Superseded by #19919 (merged) — same fix (remove dead c-S-c binding). Also competes with open #19902 and #19906.

[alt-glitch]

Superseded by #19919 (merged)

[teknium1]

Duplicate — the underlying bug was fixed on main before your PR landed.

• PR #19895 by @nftpoetrist added the try/except guard (merged 2026-05-04 21:45 UTC, commit 429b8ec).
• PR #19919 followed up by removing the dead handler entirely.

Closing issue #19903 too. Thanks for jumping on it — appreciate the quick turnaround even if we only needed one of the fixes.