Fix rangeslider "fixed" rangemode using placeholder range of autoranged counter axis

[Guhapriya01]

Fixes plotly/plotly.py#5614

Problem

With rangeslider.yaxis.rangemode: "fixed" and no explicit range, the slider renders two grey bands on initial load; toggling rangemode away and back clears them. The bands are the opp-axis masks, which shade the slider outside the fixed y-range — visible only when that range disagrees with the data.

Cause

rangeslider/defaults.js defaults the fixed range off yAxOut.range. When the counter axis autoranges, that is still the DFLTRANGEY [-1, 4] placeholder at supplyDefaults time. Unlike rangemode: 'auto', nothing revisits it afterwards.
Toggling works only because it forces a second supplyDefaults, by which point the axis holds its computed range.

Fix

Flag a fixed range defaulted off a not-yet-autoranged axis, and let doAutoRange fill it in via the path 'auto' already uses. The flag is deleted once consumed, so fixed resolves once and then stays put, and the internal key is not copied out to layout.

No schema change; auto, match, explicit ranges and non-autoranged axes are unaffected.