feat(bill_payments): validate and canonicalize bill tags

[Cedarich]

Description

Implements tag content validation and canonicalization for the Bill Payments contract, ensuring consistency with the Savings Goals contract. This change enables predictable off-chain indexing/search behavior and rejects potentially malicious tag strings.

Changes Made

• ✅ Tag validation function with charset restrictions (a-z, 0-9, hyphens, underscores)
• ✅ Canonicalization pipeline: trim whitespace → lowercase → validate
• ✅ Comprehensive test coverage for edge cases:
  • Valid tags: rent-2024, utilities, insurance_premium
  • Invalid tags: tags with special characters, excessive length, whitespace-only
  • Canonicalization verification: Rent-2024 → rent-2024
• ✅ Integration with existing bill creation/update flows
• ✅ Updated documentation in OWNER_INDEX_DESIGN.md

Requirements Met

• Secure implementation with input sanitization
• Test coverage ≥ 95%
• Consistent with savings goals tagging behavior
• Clear documentation for consumers
• Efficient and reviewable changes

Testing

cargo test -p bill_payments
# All tests passing, coverage report available

closes #485