Warn when import-mapping keys are JSON pointers instead of document paths #2459

In #2078 a user wrote:

import-mapping:
  "#/components/schemas": mymodule/internal/dto

expecting internal component refs to be remapped to another Go package. import-mapping keys are external document paths (files or URLs), not JSON pointers, so an entry like this silently never matches anything — the user just sees unresolved types with no hint about why.

Two small improvements:

• Configuration validation should reject (or warn on) import-mapping keys starting with #, with a message along the lines of "import-mapping keys are document paths; internal refs cannot be remapped — see the import-mapping examples".
• The README section on import-mapping should state explicitly that keys are document paths, that internal $refs always resolve to the current package, and point at examples/import-mapping/ for the two supported layouts (separate packages via spec split, same package via the "-" mapping).