fix: escape special characters in Linear GraphQL queries (#13490)

Creating a Linear issue from Chatwoot fails with a GraphQL parse error
when the title, description, or search term contains double quotes. For
example, a description like `the sender is "Bot"` produces this broken
query:

```graphql
issueCreate(input: { description: "the sender is "Bot"" })
```

Linear's API rejects this with `Syntax Error: Expected ":", found
String`. This affects issue creation, issue linking, and issue search —
any flow where user-provided text is interpolated into a GraphQL query.

The `graphql_value` helper was only escaping newlines (`\n`) but not
quotes, backslashes, or other characters that are meaningful inside a
GraphQL string literal. On top of that, `issue_link` and `search_issue`
bypassed `graphql_value` entirely, using raw string interpolation
instead.

The fix replaces the manual `gsub` escaping with Ruby's `to_json`, which
produces a properly escaped, double-quoted string that handles all
special characters. This is a minimal, well-understood substitution —
`to_json` on a Ruby string returns a valid JSON string literal, which is
also a valid GraphQL string literal since GraphQL uses the same escaping
rules. The `issue_link` mutation and `search_issue` query are updated to
route their parameters through `graphql_value` instead of raw
interpolation.

The `team_entities_query` and `linked_issues` methods in `queries.rb`
also use raw interpolation, but their inputs are system-generated IDs
and URLs rather than user-provided text, so they're left as-is to keep
this change focused.

lib/linear/mutations.rb

@@ -2,13 +2,10 @@ module Linear::Mutations
   def self.graphql_value(value)
     case value
     when String
-      # Strings must be enclosed in double quotes
-      "\"#{value.gsub("\n", '\\n')}\""
+      value.to_json
     when Array
-      # Arrays need to be recursively converted
       "[#{value.map { |v| graphql_value(v) }.join(', ')}]"
     else
-      # Other types (numbers, booleans) can be directly converted to strings
       value.to_s
     end
   end
@@ -47,7 +44,7 @@ module Linear::Mutations
 
     <<~GRAPHQL
       mutation {
-        attachmentLinkURL(url: "#{link}", issueId: "#{issue_id}", title: "#{title}"#{user_params_str}) {
+        attachmentLinkURL(url: #{graphql_value(link)}, issueId: #{graphql_value(issue_id)}, title: #{graphql_value(title)}#{user_params_str}) {
           success
           attachment {
             id

lib/linear/queries.rb

@@ -48,7 +48,7 @@ module Linear::Queries
   def self.search_issue(term)
     <<~GRAPHQL
       query {
-        searchIssues(term: "#{term}") {
+        searchIssues(term: #{Linear::Mutations.graphql_value(term)}) {
           nodes {
             id
             title