DEV: Strictly filter tag search limit parameter input (#21524)

### What is the problem?

It is possible to pass an arbitrary value to the limit parameter in `TagsController#search`, and have it flow through `DiscourseTagging.filter_allowed_tags` where it will raise an error deep in the database driver. MiniSql ensures there's no injection happening, but that ultimately results in an invalid query.

### How does this fix it?

This change checks more strictly that the parameter can be cleanly converted to an integer by replacing the loose `#to_i` conversion semantics with the stronger `Kernel#Integer` ones.

**Example:**

```ruby
"1; SELECT 1".to_i
#=> 1

Integer("1; SELECT 1")
#=> ArgumentError
```

As part of the change, I also went ahead to disallow a limit of "0", as that doesn't seem to be a useful option. Previously only negative limits were disallowed.

spec/requests/tags_controller_spec.rb

@@ -1112,6 +1112,15 @@ RSpec.describe TagsController do
         )
       end
 
+      it "returns error 400 for suspicious limit" do
+        get "/tags/filter/search.json", params: { q: "", limit: "1; SELECT 1" }
+
+        expect(response.status).to eq(400)
+        expect(response.parsed_body["errors"].first).to eq(
+          I18n.t("invalid_params", message: "limit"),
+        )
+      end
+
       it "includes required tag group information" do
         tag1 = Fabricate(:tag)
         tag2 = Fabricate(:tag)