feat: add optional note field to line items

[hemanth]

Closes #349

Summary

Adds an optional note string field (max 500 chars) to the line item schema, allowing buyers to attach free-text per-item instructions.

Use cases

• Food ordering: "no onions", "extra sauce"
• Gift wrapping / engraving instructions
• Voice agent scenarios: "make that a gift", "hold the pickles"

Changes

• source/schemas/shopping/types/line_item.json: Added note property with maxLength: 500 and appropriate ucp_request annotations

Design decisions

• Plain text only — structured customization belongs in catalog item variants
• 500-char limit prevents abuse while covering reasonable instructions
• Business echoes the note in responses (confirming receipt)
• Business MAY forward to downstream fulfillment via order webhooks

[jamesandersen]

Hi @hemanth - thanks for the PR. JFYI - there are two other related PRs at play:

• feat(shopping): Add additional fields proposal (from Shopify) #481 would provide more structured "additional fields" e.g. for the gift wrapping use case
• feat!: Support fractional quantities and location context for grocery shopping use cases  #597 - I just suggested something similar to support the grocery use case ... I'll cross ref your PR there as well