fix: handle sequence=0 during cluster shard failover

[Kaustubh1204]

Problem

Failover fails when a shard has never received any writes and its sequence number is 0.
In this case, the validation logic incorrectly rejects the failover even when both the
current master and the new master have a sequence of 0.

This scenario is reported in issue #366 and can occur for empty or newly created shards.

Solution

Update the failover validation logic to explicitly allow sequence == 0 when the
current master’s sequence is also 0.

This ensures failover works correctly for empty shards without changing behavior
for non-zero sequence cases.

Scope

• Limited to failover logic
• Single focused change
• No refactors, exports, CI, or unrelated fixes

Testing

• Existing tests pass
• No behavioral change for non-zero sequences

Fixes #366

[Kaustubh1204]

@git-hulk I’ve reworked the fix for #366 in a clean branch with a single focused commit.
The previous PR (#373) was closed to avoid confusion and mixed changes.