fix(group): fix(group): reject string representations of zero

[daixiheguu]

This PR fixes zero-value validation in Group.addMember() and

Description

This PR fixes zero-value validation in Group.addMember() and Group.addMembers().

The previous implementation compared the raw BigNumber input against 0n and "0" before converting it to bigint. As a result, strings such as "00", "0x0", "-0" and whitespace-padded zero values bypassed the check and were inserted as 0n.

The inputs are now normalized with BigInt() before validation and insertion.

Batch inputs are fully normalized and validated before the tree is modified.

The Group constructor remains unchanged because it intentionally accepts 0n when reconstructing groups containing removed leaves.

Related Issue(s)

Fixes #1091

Other information

Tests were added for alternative string representations of zero in both addMember() and addMembers(). The batch test also verifies that the group is not partially modified when validation fails.

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have run yarn format and yarn lint without getting any errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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