test(e2e): fix pdf-server screenshot flake from random viewUUID

[ochafik]

Problem

The e2e job is failing on main since #506 merged.

Root cause: display_pdf now returns "PDF opened. viewUUID: <random-uuid>..." in its text content. The basic-host Tool Result panel shows the first 100 chars of the JSON-serialized result as a preview, which now includes the random UUID — so pdf-server.png can never match across runs.

Before #506 the result text was just "Displaying PDF: <url>" (the viewUUID lived only in _meta, past the 100-char preview cutoff).

Fix

Add pdf-server to HOST_MASKS to mask the collapsiblePanel elements (Tool Input/Result). Same pattern used by integration, basic-*, and system-monitor servers that have dynamic result content.

Next step

After this PR's CI runs, comment /update-snapshots to regenerate pdf-server.png — adding a mask changes the screenshot (masked regions render as pink boxes), plus #506 also changed the fullscreen button from ⛶ to an SVG icon.

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commit: f351976