Add review stage coverage and fix XML tax deduplication

- Reviewer now sends all pages to the review VLM alongside extracted JSON.

- XML builder deduplicates account.tax records by rate.

- XML builder raises on confidence below min_confidence_threshold.

- Adds fractional tax-rate support with stable external IDs.

- Adds expected_invoice.xml fixture, reviewer tests, and XML regression tests.

tests/fixtures/expected_invoice.xml

@@ -0,0 +1,33 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<odoo>
+  <data noupdate="0">
+    <record id="vendor_acme_supplies_078953f5" model="res.partner">
+      <field name="name">Acme Supplies</field>
+      <field name="supplier_rank">1</field>
+      <field name="vat">GB123456789</field>
+    </record>
+    <record id="purchase_vat_20" model="account.tax">
+      <field name="name">Purchase VAT 20%</field>
+      <field name="amount">20</field>
+      <field name="amount_type">percent</field>
+      <field name="type_tax_use">purchase</field>
+    </record>
+    <record id="invoice_inv_2024_001_20240501_8047d436" model="account.move">
+      <field name="move_type">in_invoice</field>
+      <field name="partner_id" ref="vendor_acme_supplies_078953f5"/>
+      <field name="invoice_date">2024-05-01</field>
+      <field name="date">2024-05-01</field>
+      <field name="invoice_date_due">2024-06-01</field>
+      <field name="ref">INV-2024-001</field>
+      <field name="currency_id" ref="base.EUR"/>
+      <field name="narration">Net 30</field>
+    </record>
+    <record id="invoice_inv_2024_001_20240501_8047d436_line_1" model="account.move.line">
+      <field name="move_id" ref="invoice_inv_2024_001_20240501_8047d436"/>
+      <field name="name">Consulting services</field>
+      <field name="quantity">10.00</field>
+      <field name="price_unit">100.00</field>
+      <field name="tax_ids" eval="[(6, 0, [ref('purchase_vat_20')])]"/>
+    </record>
+  </data>
+</odoo>