Fix reader serialization for old clients.

[KiterLuc]

Old clients might submit a query using the legacy reader when the server
would select the refactored ones. This causes au issue because the read
state gets deserialized by the client and used to determine query
completion. The server needs to serialize a read state that the client
will understand, so for now, if the client decides to use the legacy
reader, the server needs to process that query with the legacy reader.

Long term, we will want to serialize a member of the query to determine
completion, and the read state will be serialized as a blob not
deserialized by the client. That way, the server can decide to use
whatever reader it wants.

---

TYPE: IMPROVEMENT
DESC: Fix reader serialization for old clients.

[shortcut-integration]

This pull request has been linked to Shortcut Story #20085: Segfault in core caused by VCF.

[ihnorton]

Would be great to add a test if there's a straightforward way to do that. Otherwise LGTM.

[KiterLuc]

@ihnorton I removed the backport to 2.11 as it would require to backport another, more elaborate PR that was merged in 2.12.

[ihnorton]

Thanks for adding the test 👍

[github-actions]

The backport to release-2.11 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release-2.11 release-2.11
# Navigate to the new working tree
cd .worktrees/backport-release-2.11
# Create a new branch
git switch --create backport-3446-to-release-2.11
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 d4a41b41fd6c06f1db2673fbd499fb9cbd2a71d2
# Push it to GitHub
git push --set-upstream origin backport-3446-to-release-2.11
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-2.11

Then, create a pull request where the base branch is release-2.11 and the compare/head branch is backport-3446-to-release-2.11.