[Invoices]: Implement FatturaPaHandler XML generation (FatturaPA 1.2 — Italy)

[nielsdrost7]

Context

Sub-story of #767. FatturaPaHandler (FatturaPA 1.2 — Italy) currently throws RuntimeException('... not yet implemented ...') instead of generating real XML.

Recommended package

No clearly dominant package found. Candidates surveyed: robertogallea/php-fatturapa, s2software/fatturapa, fatturaelettronicaphp/fattura-elettronica, axiostudio/fattura-elettronica. None showed the same strength-of-signal (install counts, maintenance activity) as num-num/ubl-invoice or josemmo/facturae-php in this pass — figures need verifying before picking one.

Scope

1. Research spike: properly compare the candidate packages (install counts, last release date, open issues, license) and pick one — don't default to the first result
2. Implement FatturaPaHandler::generateXml() using the chosen package

Acceptance criteria

• Research spike findings documented, package chosen with rationale
• FatturaPaHandler::generateXml() returns real, well-formed FatturaPA 1.2 XML
• Output passes PeppolXmlValidator
• Unit test asserts well-formedness and key fields, following the PeppolBisHandlerTest pattern
• FormatHandlersTest's unimplemented-formats test is updated to drop FatturaPA from the list (this includes the dedicated FatturaPaHandlerTest, currently asserting the not-yet-implemented RuntimeException)

Sequencing note

Needs its own research spike before implementation — no clear package winner yet identified. Recommend doing this last among the 8 sub-stories. See #767 for full package-research context.