Cleanup mbgl/actor/mailbox* implementation for repetition in ensuring valid weakScheduler exists before usage

[bilal614]

Ensure that valid weakScheduler exists before using it to schedule. Remove repetition of weakScheduler guarantee such that it is captured in a method.

[bilal614]

Relates to Issue #2732

[github-actions]

Bloaty Results (iOS) 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  -0.0%     -96  [ = ]       0    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-2733-compared-to-main.txt

[github-actions]

Bloaty Results 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0% +2.30Ki  +0.0%    +288    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-2733-compared-to-main.txt

Compared to d387090 (legacy)

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +27% +31.0Mi  +420% +25.1Mi    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-2733-compared-to-legacy.txt

[github-actions]

Benchmark Results ⚡

Benchmark                                                     Time             CPU      Time Old      Time New       CPU Old       CPU New
------------------------------------------------------------------------------------------------------------------------------------------
OVERALL_GEOMEAN                                            +0.0098         +0.0098             0             0             0             0

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/benchmark-results/pr-2733-compared-to-main.txt

[bilal614]

@mwilsnd: Could you please have a look at the changes for providing any review feedback. thanks

[bilal614]

@louwers: The pipeline looks green. Is there any review feedback I can incorporate before the merge?

[louwers]

Nope, let's give @mwilsnd one more day to look at it since he reworked this code recently.

Otherwise I'll merge it later today!

[mwilsnd]

Looks good, thanks for consolidating this stuff!