fix(web): keep task pricing editable after a plugin narrows its enum values

When a task plugin stops declaring an enum value (Seedance 2.0 mini now
offers 480p and 720p only), an expression saved with 1080p and 4k
branches no longer opened in the visual price table: the parser rejected
the whole tier chain on the first undeclared value, the editor fell back
to expression mode with a "cannot be represented" notice, and switching
to the visual editor reset every price to zero.

Treat a branch on an undeclared enum value as unreachable and skip it,
so the remaining tiers stay editable with their prices preserved. Nested
conditions in the pricing display treat such a comparison as never
matching. Editing a price regenerates the expression without the
unreachable branches; opening the editor alone leaves the stored
expression unchanged.