Fix policy deletion with commas in quoted fields (#1733)

[Jbansal2]

This fixes an issue where policies with commas in quoted fields (e.g., ABAC conditions) were not being deleted correctly in batch operations.

Root Cause:

• Policy hash keys were generated using comma as separator via strings.Join(rule, ',')
• This created hash collisions when policy fields contained commas
• Example: ['alice', 'data,file', 'read'] and ['alice', 'data', 'file', 'read'] produced the same hash key 'alice,data,file,read'

Solution:

• Introduced policyKey() helper function that uses ASCII Unit Separator (\x1f)
• This character is designed for field separation and won't appear in policy data
• Updated all policy operations to use policyKey() instead of strings.Join()

Changes:

• model/policy.go: Added policyKey() and replaced all hash key generation
• Added comprehensive tests demonstrating the fix
• All existing tests pass (216 tests)

Fixes #1733