fix: Change messages.source_id to text column (#12908)

## Summary
Changes `messages.source_id` from `string` (255 char limit) to `text`
(20,000 char limit) to support long email Message-ID headers.

## Changes
- Migration to change column type from string to text
- Added spec tests for source_id length validation

## Related
Fixes
https://linear.app/chatwoot/issue/CW-5961/activerecordrecordinvalid-validation-failed-source-is-too-long-maximum

db/migrate/20251119161025_change_messages_source_id_to_text.rb

@@ -0,0 +1,9 @@
+class ChangeMessagesSourceIdToText < ActiveRecord::Migration[7.1]
+  def up
+    change_column :messages, :source_id, :text
+  end
+
+  def down
+    change_column :messages, :source_id, :string
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.1].define(version: 2025_11_14_173609) do
+ActiveRecord::Schema[7.1].define(version: 2025_11_19_161025) do
   # These extensions should be enabled to support this database
   enable_extension "pg_stat_statements"
   enable_extension "pg_trgm"
@@ -954,7 +954,7 @@ ActiveRecord::Schema[7.1].define(version: 2025_11_14_173609) do
     t.datetime "updated_at", precision: nil, null: false
     t.boolean "private", default: false, null: false
     t.integer "status", default: 0
-    t.string "source_id"
+    t.text "source_id"
     t.integer "content_type", default: 0, null: false
     t.json "content_attributes", default: {}
     t.string "sender_type"