Add a kicad_export checkbox to parameters, allowing users to control
which specifications appear as fields in the KiCad HTTP library API.
Parameters with kicad_export enabled are included using their formatted
value, without overwriting hardcoded fields like description or Stock.
- Add comprehensive KiCadHelperTest with 14 test cases covering:
- Stock quantity calculation (zero, single lot, multiple lots)
- Stock exclusion of expired and unknown-quantity lots
- Storage location display (present, absent, multiple)
- Datasheet URL resolution by type name, attachment name, PDF extension
- Datasheet fallback to Part-DB URL when no match
- "Data sheet" (with space) name variant matching
- Fix PDF extension detection for external attachments (getExtension()
returns null for external-only attachments, now also parses URL path)
