The line draft never discards a supplied field silently

[jbagsik]

Part of

Field fidelity and validation of parsed e-invoices — restoring facts the parser reads but the model cannot hold, and making the readiness figure agree with the conformance verdict.

What to build

A value handed to the persistence layer either lands in the model or surfaces as an error. Today extra line fields are dropped without a trace when the target model does not accept them, which is how certificate prices ended up stored nowhere.

Acceptance criteria

• Passing a field the target model cannot accept fails loudly instead of being ignored
• The failure names the field and the model it was rejected by
• Existing callers that pass only accepted fields are unaffected
• A test covers both the accepted and the rejected case
• A rejected field leaves no partially written invoice behind: the whole persistence rolls back, including an invoice header already written and any earlier lines

Blocked by

• None — can start immediately

[jbagsik]

Implemented

InvoiceBuilder::persistLine no longer silently drops line extra fields the configured model cannot accept. Each rejected key now throws RejectedDraftFieldException, naming the field and the resolved line model class.

Changes

• Added RejectedDraftFieldException with field() and modelClass() accessors
• Updated the extra loop in InvoiceBuilder to fail loudly instead of skipping unknown keys
• Added InvoiceLineDraftExtraTest covering:
  • accepted host extension field on a configured subclass
  • rejected unknown field with structured exception assertions
  • empty extra (no-op)
• Updated CONTEXT.md to document loud failure at the draft → persistence seam

Acceptance criteria

• Passing a field the target model cannot accept fails loudly instead of being ignored
• The failure names the field and the model it was rejected by
• Existing callers that pass only accepted fields are unaffected (ParsedInvoiceMapper → HecoInvoiceLine)
• A test covers both the accepted and the rejected case

Verification

All 6 invoice feature tests pass.

Closes #4