fix(agent): surface err.cause in uploadFromUrl tool error messages

[giladresisi]

Why

undici's fetch() rejects network-level failures (DNS, TLS, connection refused, SSRF-dispatcher block) with a generic TypeError('fetch failed') and puts the actual reason in err.cause — see lib/web/fetch/index.js#L271:

p.reject(new TypeError('fetch failed', { cause: response.error }))

The uploadFromUrl tool's catch block (added in #1649) only surfaced err.message, so the agent — and the user — saw a bare fetch failed with no indication of why the URL couldn't be fetched.

What

Append err.cause.message to the { error } result when present, e.g. fetch failed (getaddrinfo ENOTFOUND example.com) instead of just fetch failed. Error.cause isn't in the es2020 lib typings this repo compiles against, hence the cast; the access is guarded so a missing or empty cause changes nothing.

Reproduction & validation

Pre-fix: a fetch rejection surfaced only as the bare Failed to upload media from URL: fetch failed.

Post-fix, live via the uploadFromUrl agent tool over MCP:

http://no-such-host-cause-test.invalid/a.jpg
  ->  {"error":"Failed to upload media from URL: fetch failed (getaddrinfo ENOTFOUND no-such-host-cause-test.invalid)"}

Together with the companion SSRF literal-IP fix (#1671) — the two compose to give an actionable reason for blocked internal addresses:

http://192.168.1.1/a.jpg
  ->  {"error":"Failed to upload media from URL: fetch failed (Blocked IP)"}

Relation to #1671

Independent files, independently mergeable. This PR makes #1671's SSRF block (and every other fetch rejection on this path) legible in the agent's message; #1671 stands on its own without it.

🤖 Generated with Claude Code

[postiz-contribution]

Contribution-checker quality warning
Heuristic score: 10/100 (low). This is a non-blocking warning surfaced by the project's quality settings.

Heuristics that flagged:

• Excessive inline code references: 15 inline refs (>3)
• PR doesn't use the repo's PR template: 5 required checkbox items missing (max 1, match ≥80%)
• Body adds too many extra headers beyond the template: 4 extra headers (>1)
• AI watermark phrase: Matched: "Generated with Claude Code"
• Commit message too long: Longest: 1370 chars

If this is a genuine contribution, please add detail to your PR description and tighten the diff scope before reviewers look at it.

[postiz-agent]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues
✅	Code Security	0	0	0	0	0 issues

💻 Catch issues earlier using the plugins for VS Code, JetBrains IDEs, Visual Studio, and Eclipse.