fix(mcp): over-long reason gets VALIDATION_ERROR and a specific Swedish message (#1811)

* fix(mcp): over-long reason gets VALIDATION_ERROR and a specific Swedish message

gnubok_reverse_journal_entry / gnubok_undo_sie_import cap `reason` at 500
characters, but exceeding it produced code UNKNOWN_ERROR with message_sv
"Något gick fel. Försök igen." while the cause sat only in message_en.
getStructuredError now infers VALIDATION_ERROR from the message and
getErrorMessage maps it to "Motiveringen får vara högst 500 tecken.".

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(errors): pin the reason-length pattern to 500 and assert the envelope on undo_sie_import

Review findings: the Swedish message hard-codes 500, so the pattern must
match that limit only; the undo_sie_import 501-char test now asserts code
and both localized messages like the reverse test does.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/errors/__tests__/get-structured-error.test.ts

@@ -35,6 +35,12 @@ describe('getStructuredError', () => {
     expect(result.remediation?.tool).toBe('gnubok_lock_period')
   })
 
+  it('maps an over-long reason to VALIDATION_ERROR with a specific Swedish message', () => {
+    const result = getStructuredError(new Error('reason must be 500 characters or fewer'))
+    expect(result.code).toBe('VALIDATION_ERROR')
+    expect(result.message_sv).toBe('Motiveringen får vara högst 500 tecken.')
+  })
+
   it('infers PERIOD_HAS_UNBOOKED_TRANSACTIONS from Swedish lock-error message', () => {
     const result = getStructuredError(
       new Error('Kan inte låsa period: 3 affärstransaktion(er) saknar bokföring.')

lib/errors/get-error-message.ts

@@ -98,6 +98,10 @@ const GENERIC_FALLBACK: Bilingual = { sv: 'Något gick fel. Försök igen.', en:
 
 // Known error patterns → user-friendly Swedish messages
 const ERROR_PATTERN_MAP: [RegExp, string | null][] = [
+  [
+    /reason must be 500 characters or fewer/i,
+    'Motiveringen får vara högst 500 tecken.',
+  ],
   [
     /locked\/closed fiscal period/i,
     'Perioden är låst. Verifikationen kan inte skapas i en stängd eller låst period.',

lib/errors/get-structured-error.ts

@@ -158,6 +158,7 @@ function inferCode(message: string): string | null {
   if (/Bokföringen är låst/i.test(message)) return 'PERIOD_LOCKED'
   if (/Transaction not found/i.test(message)) return 'NOT_FOUND'
   if (/Invoice not found/i.test(message)) return 'NOT_FOUND'
+  if (/must be \d+ characters or fewer/i.test(message)) return 'VALIDATION_ERROR'
   return null
 }