Add inbound reply simulation

[rreinhardt9]

Summary

• Phase 1 (outbound interception + viewing UI) shipped and is in use; this is phase 2, deliberately deferred until phase 1 had real usage.
• Adds SmsTrap.reply_handler — a bare callable (#call(from:, to:, text:)) the host app configures in a dev-only initializer to trigger its real inbound-webhook path when a dev submits a reply in the UI. Kept provider-agnostic and symmetric with Connector's outbound shape — sms_trap never knows what's inside the callable.
• The reply form on the conversation show page only renders when SmsTrap.reply_handler? is true — apps that never need inbound simulation see no dead/disabled UI. Reading reply_handler unset raises SmsTrap::ReplyHandlerNotConfigured (loud, immediate, no silent no-op — matches the gem's existing style).
• README's "Roadmap" section replaced with a real "Simulating inbound replies" section documenting the contract and a wiring example.

Incidental fix

test/dummy/config/environments/test.rb had config.action_dispatch.show_exceptions = false (a boolean). Rails 7.2's ExceptionWrapper#show? only treats the literal symbol :none as "raise instead of render" — the boolean silently fell through to the render branch, swallowing exceptions that integration tests expect to propagate via assert_raises. Fixed to :none, matching the file's own comment ("Raise exceptions instead of rendering exception templates").

Test plan

• bin/rails test — 18 runs, 0 failures
• bundle exec rubocop — clean
• Manually exercised the full flow against the running dummy app via curl (browser extension wasn't available in this session): empty state → send outbound → conversation list/show render it → reply form present (dummy initializer configures a stand-in handler) → submit reply → inbound bubble appears → dummy handler's log line confirms it was invoked with the correctly-derived from/to (their_number → our_number)