fix: accept Pandoc image alt metadata in LaTeX

[aryansk]

Closes #2275

Summary

• Define a no-op Gin alt key so Pandoc-generated alt={...} image metadata is accepted by graphicx/adjustbox instead of failing PDF compilation. Adds a template regression check; the shim was also sanity-compiled with pdflatex.
• keep the change narrowly scoped to the reported behavior
• add focused regression coverage

Validation

• checked the reported failure against current main
• patch is based on current main
• full repository tests should run in CI