Fix external images when local media caching is disabled

[brozkeff]

Summary

• Preserve the original external image URL when DokuWiki media caching is disabled or fails.
• Pass the resolved target through both mPDF asset path arguments.
• Add a regression test using an ownCloud-style external image URL.

This restores the fallback behavior lost during the mPDF 8.2 migration.

Fixes #562.

Developed and diagnosed with assistance from GPT-5.6-sol.

Test plan

• External images render in generated PDFs with fetchsize = 0.
• PHP syntax checks pass.
• Git diff validation passes.

[splitbrain]

no. the issue is that media_get_from_URL() doesn't do what it says when fetch size is 0. you pass the URL on to mpdf which just hands it back to us to be downloaded. all that is needed is to internally set fetchsize here and make the code work as intended.

[brozkeff]

OK, I already see the the proper path at #564 and can confirm that PR works fine.