feat: add BT-120 VAT exemption reason (backport #247)

[mergify]

Adds a BT-120 textfield for the VAT exemption reason to the E-Invoice Settings: #183

---

This is an automatic backport of pull request #247 done by Mergify.

[greptile-apps]

Confidence Score: 5/5

This PR is safe to merge; it adds a new optional field with purely additive changes and no breaking modifications to existing logic.

The change is strictly additive: a new nullable text field in settings and two child-doctype columns. BT-120 is only written when the field is non-empty and only alongside an already-emitted BT-121, which matches the EN16931 constraint. The import path correctly guards the None case. The defensive getattr fix for supplier_numbers is a safe improvement. No migration or schema-breaking change is required for existing installs because all new fields are optional/nullable.

No files require special attention.

Sequence Diagram

sequenceDiagram
    participant EIG as EInvoiceGenerator
    participant DB as frappe.db
    participant TT as ApplicableTradeTax

    EIG->>DB: get_single_value("E Invoice Settings", "vat_exemption_reason_text")
    DB-->>EIG: "vat_exemption_reason_text (str | None)"

    alt "Item-level tax, rate == 0"
        EIG->>TT: set exemption_reason_code (BT-121)
        EIG->>TT: _set_optional_vat_exemption_reason_text → exemption_reason (BT-120)
    end

    alt No taxes on document (_add_empty_tax)
        EIG->>TT: set exemption_reason_code (BT-121)
        EIG->>TT: _set_optional_vat_exemption_reason_text → exemption_reason (BT-120)
    end

    note over EIG,TT: BT-120 only written when vat_exemption_reason_text is non-empty

    participant EII as EInvoiceImport
    participant XML as Parsed CII XML

    XML-->>EII: tax.exemption_reason._text
    EII->>EII: store as vat_exemption_reason_text on E Invoice Trade Tax row

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_{Reviews (2): Last reviewed commit: "Merge remote-tracking branch 'origin/ver..." | Re-trigger Greptile}

[barredterra]

@greptileai

[barredterra]

Tested, LGTM