fix(portal): ignore prose in snip Seen-on values

[KB-x]

Why

Some snip headers use prose to indicate that a snip doesn't apply to one OS, e.g. Junos: (none in this JVD — Junos PEs use ...) or Junos: —. The parser was splitting the value on whitespace and turning every word into a device chip, producing nonsense like in this in this next to the real device tokens in the Snip Library UI.

What

• portal/scripts/generate-snips.mjs — when parsing Seen on: values, stop at the first (...) continuation note and skip tokens that don't start with an alphanumeric character.
• portal/src/data/snips.json — regenerated; device chips now show only real device names.

Expected impact

• Snip Library detail panes display clean Seen on device lists.
• No content changes to the underlying snip files were required.

Source-of-truth

snips.json is generated. Regenerated via node portal/scripts/generate-snips.mjs; CI --check will validate freshness.

Test

• Ran the generator locally — 320 snips, 0 warnings.
• Verified 0 prose tokens remain in any seenOn.junos / seenOn.evo array.
• Spot-checked enterprise_wan/ewan_adv_core_edge/.../evpn-vpws-fxc.conf (the originally reported snip) — chips now show only the four real devices.