Add property based convergence test on Json.Object

[Yeongeunn]

Description:

Related to #1288.

Add property-based convergence tests for Json.Object.

The test will:

• reuse the PBT helper introduced in Add property based convergence test on Json.Counter #1300;
• set up multiple clients and documents with manual sync;
• generate Object assignment and deletion operations with intermediate syncs;
• run the final sync drain after the generated operations finish;
• assert that all replicas have identical toSortedJSON() output;
• report reproducible failures using the fast-check seed and path.

For the initial scope, the test will use root-level keys and primitive JSON
values (number, string, boolean, and null). Nested Object and Array
values are out of scope for this first target.

Why:

Existing Object integration tests cover hand-written concurrent scenarios,
but cannot explore many combinations of client selection, keys, values,
operations, and sync ordering.

Property-based tests can generate these combinations and verify that all
replicas converge after concurrent Object assignment and deletion operations.

[ggyuchive]

Hi @Yeongeunn , thanks for open this issue!
Assigned.
+) When you find divergence failure during PBT run, also feel free to open issue with simplified scenario.

[Yeongeunn]

Thank you for assigning me! I'll start working on it. If I find a divergence during the PBT run, I'll simplify the reproduction scenario and open a separate issue.