fix(captain): localize AI summary to account language (#13790)

AI-generated summaries now respect the account's language setting.
Previously, summaries were always returned in English regardless of the
user's configured language, making section headings like "Customer
Intent" and "Action Items" appear in English even for non-English
accounts.

Previous behavior:
<img width="1336" height="790" alt="image"
src="https://github.com/user-attachments/assets/5df8b78b-1218-438d-9578-a806b5cb94ac"
/>


Current Behavior: 
<img width="1253" height="372" alt="image"
src="https://github.com/user-attachments/assets/ae932c97-06da-4baf-9f77-9719bc9162e8"
/>


## What changed
- Added explicit account locale to the AI system prompt in
`Captain::SummaryService`
- Updated the summary prompt template to instruct the model to translate
section headings

## How to test
1. Configure an account with a non-English language (e.g., Portuguese)
2. Open a conversation with messages
3. Use the Copilot "Summarize" feature
4. Verify that section headings ("Customer Intent", "Conversation
Summary", etc.) appear in the account's language

---------

Co-authored-by: Aakash Bakhle <48802744+aakashb95@users.noreply.github.com>

spec/lib/captain/summary_service_spec.rb

@@ -35,7 +35,8 @@ RSpec.describe Captain::SummaryService do
       expect(service).to receive(:make_api_call) do |args|
         expect(args[:messages].length).to eq(2)
         expect(args[:messages][0][:role]).to eq('system')
-        expect(args[:messages][0][:content]).to eq('Summarize this')
+        expect(args[:messages][0][:content]).to include('Summarize this')
+        expect(args[:messages][0][:content]).to include("Reply in #{account.locale_english_name}")
         expect(args[:messages][1][:role]).to eq('user')
         expect(args[:messages][1][:content]).to be_a(String)
         { message: 'Summary' }