Cap prediction cone to shortest predBG array length

[quarktwain]

Fixes #637.

Summary

updateOpenAPSPredictionDisplay() capped the cone at the longest predBG
array, so at indices past shorter arrays the cone deformed -- the band
narrowed or shifted as contributing arrays dropped out one by one.

This caps at the shortest array length instead. Every cone point is
computed from the same set of contributing arrays; the band ends cleanly
when the first array runs out.

Compared against nightscout/Trio Trio/Sources/Modules/Home/HomeStateModel+Setup/ForecastSetup.swift:

minCount = max(12, allForecastValues.map(\.count).min() ?? 0)
let localMinCount = minCount
// ...
let minForecast = (0 ..< localMinCount).map { ... }
let maxForecast = (0 ..< localMinCount).map { ... }

Trio iterates 0 ..< localMinCount -- every cone point uses values from
all contributing arrays. This PR brings LoopFollow to the same shape.

Did not adopt Trio's max(12, ...) floor; open to adding it if preferred.

User-visible behavior

• For users whose shortest predBG array is materially shorter than the
  longest, the cone will visibly end earlier than before. The
  previously-rendered tail was narrowing/shifting with no real meaning.
• topPredictionBG (graph y-axis upper bound) is updated inside the
  loop. With the shorter loop, the upper bound may sit slightly tighter
  for some sessions. No data is cropped (the loop's output is what gets
  drawn); just less empty headroom in mixed-length-array cases.

[github-actions]

⚠️ This PR targets the main branch. We do not accept PRs directly to main — please retarget your PR to the dev branch instead.

[quarktwain]

Retargeted to dev -- thanks for the auto-warning. Branch is clean against dev (no rebase needed; main and dev are at the same commit currently).

[dnzxy]

+1 on this one. Reported the same quirk (#637 ) privately to the LF team.

[marionbarker]

Test

✅ successful test

Configuration

Use an iPhone SE running iOS 26.5.
Build LoopFollow dev and this branch
Observe a Trio URL

Screenshots

These screenshots demonstrate this PR (middle screen in graphic) fixes the distorted cone (right screen in graphic is dev branch).

Graphic has forecast set as cones for Trio (left screen) and LoopFollow.

[marionbarker]

Approve from code review and test.

[marionbarker]

Thanks for the close/reopen - that fixed the test that was stuck.