fix(media): require LinkedIn permalink IDs

[alectimison-maker]

Summary

• require a non-empty post identifier after LinkedIn /posts/ and /feed/update/ prefixes
• route empty LinkedIn container paths through the existing screenshot/permalink-resolution flow
• preserve direct-download behavior for both supported LinkedIn permalink shapes in Chrome and Firefox

Problem

isDirectPublicMediaUrl treated the bare /posts/ and /feed/update/ paths as concrete public media items. Recommended downloads therefore skipped visual target resolution and could send a non-item URL to download_public_media.

LinkedIn's documented /feed/update/<activityUrn> permalink shape requires a dynamic identifier: https://learn.microsoft.com/en-us/linkedin/marketing/community-management/shares#constructing-permalinks

Testing

• node test/run.js: 1449 passed, 1 pre-existing repository failure (package.json is 26.0.10, newest CHANGELOG.md entry is 26.0.0)
• node test/security/injection-corpus.mjs: 60/60 checks passed
• verified empty /posts/ and /feed/update/ paths start with screenshot preflight
• verified /posts/<id> and /feed/update/<activityUrn> retain the direct fast path
• Chrome/Firefox URL helpers are byte-identical

Scope

This only tightens LinkedIn's two existing permalink patterns; no other host recognition or download behavior changes.

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[Copilot]

Pull request overview

Tightens LinkedIn permalink detection so container paths require visual resolution while concrete posts retain direct downloads.

Changes:

• Require identifiers after both supported LinkedIn permalink prefixes.
• Add Chrome/Firefox coverage for direct and empty permalink paths.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/run.js	Tests direct and empty LinkedIn permalinks.
src/firefox/src/agent/public-media-url.js	Requires a LinkedIn post identifier.
src/chrome/src/agent/public-media-url.js	Mirrors the Firefox URL validation change.

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