[sharedb] Add 'retry' middleware action

[dawidreedsy]

Summary

Adds the new 'retry' middleware action from share/sharedb#761 to middleware.ActionContextMap.

retry is triggered when an op loses the race to commit, before it is retried against a newer snapshot. Middleware can use it to reset state stored on the request during the failed attempt. It receives the same SubmitRequest as the other submit actions, so it is typed as SubmitContext (with no $fixup, which is only valid in 'apply').

backend.use("retry", (context, next) => {
    delete context.snapshotBeforeApply;
    next();
});

sharedb-tests.ts adds "retry" to the submit-related actions loop, plus a check that $fixup isn't allowed in it.

Draft until share/sharedb#761 is merged and released. The version in package.json will need bumping to match that release if it ships as a minor.

---

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Add or edit tests to reflect the change.
• Follow the advice from the readme.
• Avoid common mistakes.
• Run pnpm test <package to test>.

If changing an existing definition:

• Provide a URL to documentation or source code which provides context for the suggested changes: ✨ Add retry middleware action share/sharedb#761
• If this PR brings the type definitions up to date with a new version of the JS library, update the version number in the package.json. (pending the sharedb release)

cc @alecgibson @ericyhwang

🤖 Generated with Claude Code