feat: add invoice online URL and PDF download tools

[harryblam]

Summary

Add two new read-only tools for accessing Xero invoice documents:

Tool	Description
get-invoice-online-url	Get the public shareable URL for an invoice (the link customers see for online viewing/payment)
get-invoice-as-pdf	Download the Xero-rendered invoice PDF as base64

Changes

New files

• src/handlers/get-xero-invoice-online-url.handler.ts — calls accountingApi.getOnlineInvoice(), extracts URL from response
• src/handlers/get-xero-invoice-as-pdf.handler.ts — calls accountingApi.getInvoiceAsPdf(), converts Buffer to base64
• src/tools/get/get-invoice-online-url.tool.ts — tool definition with invoiceId parameter; gracefully handles invoices without online viewing enabled
• src/tools/get/get-invoice-as-pdf.tool.ts — tool definition with invoiceId parameter; returns PDF content as base64

Modified files

• src/tools/get/index.ts — registers both new tools

Scope

Both tools use accountingApi.getOnlineInvoice() and accountingApi.getInvoiceAsPdf() which fall under the existing accounting.transactions scope. No new scopes required.

Testing

Tested against a live Xero account:

• get-invoice-online-url returns the correct shareable URL (e.g. https://in.xero.com/...)
• get-invoice-as-pdf returns a valid PDF (~90KB base64 for a simple invoice)
• Invoices without online viewing enabled get a clear informative message instead of an error
• Existing tools continue to work — no breaking changes