Fix custom tag render result stringification

[sjh9714]

Summary

Fixes #1893.

Custom tags that rely on the legacy render(context) compatibility path were appending non-string render results directly to the output buffer. That made an integer render result use String#<< byte semantics, so 49 rendered as 1. This restores the pre-refactor behavior by joining array render results and stringifying other truthy render results before appending.

Changes

• Convert legacy custom tag render results before appending to the output buffer.
• Add regression coverage for integer and array render results.

Testing

• Red check on a temporary main worktree: docker run --rm -v /tmp/liquid-red-1893:/repo -w /repo ruby:4.0.2 ruby -Itest -Ilib test/integration/tag_test.rb -n /stringify/ failed with Expected: "49", Actual: "1".
• docker run --rm -v "$PWD":/repo -w /repo ruby:4.0.2 ruby -Itest -Ilib test/integration/tag_test.rb -n /stringify/
• docker run --rm -v "$PWD":/repo -w /repo ruby:4.0.2 ruby -Itest -Ilib test/integration/tag_test.rb
• docker run --rm -v "$PWD":/repo -w /repo ruby:4.0.2 ruby -Itest -Ilib test/integration/block_test.rb
• docker run --rm -v "$PWD":/repo -w /repo ruby:4.0.2 ruby -c lib/liquid/tag.rb
• docker run --rm -v "$PWD":/repo -w /repo ruby:4.0.2 ruby -c test/integration/tag_test.rb
• git diff --check

Verification gap: I also tried ruby -Itest -Ilib -S rake base_test in the Ruby 4.0.2 container, but the plain container lacked lru_redux. A disposable Bundler install was blocked because the Shopify/liquid-spec branch self-drop-env-lookup-specs could not be resolved.

Note: I used Codex while preparing this change, reviewed the final diff, and ran the listed checks locally.

[sjh9714]

I have signed the CLA!