fix: rate limit widget conversation transcript API (#15085)

## Description

The widget conversation transcript endpoint (`POST
/api/v1/widget/conversations/transcript`) has no rate limit. Every other
comparable endpoint does: the agent-facing transcript API and the widget
conversation-create and contact-update endpoints are all throttled. This
gap lets a single client trigger a large burst of transcript emails from
one conversation.

This adds an IP-based throttle (5 requests/hour) for the endpoint,
placed inside the existing widget-API throttle block so it inherits the
`ENABLE_RACK_ATTACK_WIDGET_API` opt-out used by embedded/iframe clients.
The limit is generous for legitimate use (a visitor emailing themselves
a transcript) while stopping abusive loops. Throttled requests get the
standard 429 the widget already handles.

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

`config/initializers/rack_attack.rb` throttles have no existing specs in
this file, so this follows the established convention (no new spec).
Verified `ruby -c` and `rubocop` pass on the file. The new throttle
mirrors the sibling widget throttles directly above it (same IP key,
path guard, and structure).

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] My changes generate no new warnings

---------

Co-authored-by: Sojan Jose <sojan@pepalo.com>

config/initializers/rack_attack.rb

@@ -31,10 +31,11 @@ class Rack::Attack
       (default_allowed_ips + env_allowed_ips).include?(remote_ip)
     end
 
-    # Rails would allow requests to paths with extensions, so lets compare against the path with extension stripped
-    # example /auth & /auth.json would both work
+    # Rails allows paths with extensions and trailing slashes, so compare against a normalized path.
+    # For example, /auth, /auth.json, and /auth/ should all use the same throttle.
     def path_without_extensions
-      path[/^[^.]+/]
+      normalized_path = path[/^[^.]+/]
+      normalized_path == '/' ? normalized_path : normalized_path.sub(%r{/+\z}, '')
     end
   end
 
@@ -188,6 +189,11 @@ class Rack::Attack
     throttle('widget?website_token={website_token}&cw_conversation={x-auth-token}', limit: 5, period: 1.hour) do |req|
       req.ip if req.path_without_extensions == '/widget' && ActionDispatch::Request.new(req.env).params['cw_conversation'].blank?
     end
+
+    ## Prevent Transcript Bombing on Widget API ###
+    throttle('api/v1/widget/conversations/transcript', limit: 5, period: 1.hour) do |req|
+      req.ip if req.path_without_extensions == '/api/v1/widget/conversations/transcript' && req.post?
+    end
   end
 
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