fix: escape formula characters in CSAT CSV export (#15335)

## Description

The CSAT survey response CSV export builds rows with the standard
library `CSV`, writing values verbatim, including the feedback message,
which is free text submitted by end users. When a cell begins with a
formula character (`=`, `+`, `-`, `@`, and tab/CR), spreadsheet
applications interpret it as a formula on open. This switches the three
`CSV.generate_line` calls in the export template to
`CSVSafe.generate_line` (the `csv-safe` gem already used by the v2
report exports), which prefixes such fields so they are treated as text.

No new dependency, no behavioural change beyond neutralising
formula-leading cells.

Ref https://linear.app/chatwoot/issue/CW-7473

## Type of change

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

## How Has This Been Tested?

`bundle exec rspec
spec/controllers/api/v1/accounts/csat_survey_responses_controller_spec.rb`
— added a case asserting a formula-leading feedback value is neutralised
in the downloaded CSV.

## 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
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes

app/views/api/v1/accounts/csat_survey_responses/download.csv.erb

@@ -11,7 +11,7 @@
   ]
   headers << I18n.t('reports.csat.headers.review_notes') if ChatwootApp.enterprise?
 -%>
-<%= CSV.generate_line(headers) -%>
+<%= CSVSafe.generate_line(headers) -%>
 <% @csat_survey_responses.each do |csat_response| %>
 <% assigned_agent = csat_response.assigned_agent %>
 <% contact = csat_response.contact %>
@@ -29,10 +29,10 @@
   ]
   row << csat_response.csat_review_notes if ChatwootApp.enterprise?
 -%>
-<%= CSV.generate_line(row).html_safe -%>
+<%= CSVSafe.generate_line(row).html_safe -%>
 <% end %>
 <%=
-  CSV.generate_line([
+  CSVSafe.generate_line([
     I18n.t(
       'reports.period',
       since: Date.strptime(params[:since], '%s'),