fix(jobnet-search): route external ads to jobAdUrl with http(s) validation and fallback (#538) (#557)

External ads (isExternal: true) 404 on /find-job/{jobAdId} and hit the MitID login wall on /job/{jobAdId}. Route them to their employer-provided destination in jobAdUrl when an absolute http(s) URL is present.

When jobAdUrl is missing, empty, or carries an invalid/non-http scheme, fall back to https://jobnet.dk/find-job/{jobAdId} instead of emitting an empty string so downstream /scrape deduplication preserves unique URLs. Both url and jobAdUrl validate the scheme consistently.

Observation and test cases reported by NishilRathod in #557.

.agents/skills/jobnet-search/SKILL.md

@@ -202,5 +202,5 @@ All errors are written to **stderr** as `{ "error": "...", "code": "..." }` and
 - Pagination is 1-indexed (`--page 1` is the first page).
 - `search` results omit the HTML job description — use `detail` to get it.
 - `detail --format plain` strips HTML tags for readable text output.
-- Job ad detail pages on jobnet.dk: `https://jobnet.dk/find-job/{jobAdId}`
+- Job ad detail pages on jobnet.dk: `https://jobnet.dk/find-job/{jobAdId}` (non-external ads; external ads use employer `jobAdUrl`).
 - `suggestions` is tuned for Danish job titles — English terms may return empty results.