Consolidate key-binding actions and handler dispatch

[Aditya-Dev759]

Description

• Centralized key‑binding actions into a dedicated helper for reuse
• Clarified identifier quoting logic in the SQL completer
• Centralized support info constant into the shared constants module
• Routed special command handling through dedicated handlers
• No functional behavior changes intended

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .