When a var() fallback value contained trailing whitespace (e.g.
`var(--foo, flex )`), parse_display_value() miscounted the tokens.
The remaining_token_count() check included whitespace tokens, routing
single-keyword values like "flex" to the multi-component path. The
multi-component parser then failed when encountering the trailing
whitespace token.
Fix this by counting only non-whitespace tokens for the single vs
multi-component routing decision, and by skipping whitespace in the
multi-component parsing loop.
This test shows that var() fallback values with trailing whitespace
(e.g. `var(--foo, flex )`) fail to parse correctly for the display
property, falling back to initial values instead.
