Fix quantity field encoding/decoding and add test suite

[rosskukulinski]

Summary

• Fix Item constructor to read quantity from quantityPb.amount or deprecatedQuantity fields (protobuf format from API)
• Fix Item._encode() to properly encode quantity as quantityPb.amount per the ListItem protobuf schema
• Add Mocha/Chai test framework with comprehensive integration tests

Problem

The quantity field was broken in two ways:

1. Reading: Items returned from API have quantity in quantityPb.amount or deprecatedQuantity, but the constructor only looked at quantity
2. Writing: _encode() tried to set a quantity field that doesn't exist in the protobuf schema, causing the error: .pcov.proto.ListItem#quantity is not a field

Solution

• Constructor now reads: quantityPb?.amount ?? deprecatedQuantity ?? quantity
• _encode() now writes: quantityPb: { amount: String(quantity) }

Test Coverage

Added 22 integration tests covering:

• Adding items with/without quantity
• Numeric and string quantities
• Updating quantity on existing items
• Item details, checked status, name updates
• Multiple property updates at once
• Item retrieval (by id/name)
• JSON serialization
• Input validation

Run tests with:

[email protected] ANYLIST_PASSWORD=yourpassword npm test

Test plan

• All 22 integration tests pass
• Adding items with quantity works
• Updating quantity on existing items works
• Removing items with quantity works (no more protobuf error)

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