feat: add Kosit external validator for UBL XML validation

[Copilot]

Adds support for validating UBL XML documents against a Kosit validator service via HTTP POST.

Changes

• src/validator/KositValidator.ts — Sends XML to Kosit endpoint, parses the XML report response to extract accept/reject status, errors, and warnings. Uses native fetch, no new dependencies.
• src/index.ts — Exposes validateWithKosit(xml, options?) on PeppolToolkit; re-exports validator types.
• tests/kosit-validator.test.ts — 10 tests covering accepted/rejected reports, warnings, error handling, custom endpoints.

Usage

const toolkit = new PeppolToolkit();
const xml = toolkit.invoiceToPeppolUBL(invoice);

// Uses default endpoint http://localhost:8081/
const result = await toolkit.validateWithKosit(xml);

// Or with custom endpoint
const result = await toolkit.validateWithKosit(xml, {
  endpoint: 'http://my-validator:9090/',
});

console.log(result.valid);    // boolean
console.log(result.errors);   // KositValidationMessage[]
console.log(result.warnings); // KositValidationMessage[]
console.log(result.rawXml);   // raw Kosit XML report

// Standalone usage
const validator = new KositValidator({ endpoint: 'http://localhost:8081/' });
const result = await validator.validate(xml);

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 26 complexity . 2 duplication

Metric	Results
Complexity	26
Duplication	2

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