Add configurable mmap prefetch pages support

[zeronerdzerogeekzerocool]

Ports the mmap prefetch limit from a hard-coded 100GB constant to an instance-level config (pinot.server.instance.mmap.prefetch.max.size), adapted to the current IndexLoadingConfig/SegmentDirectoryLoaderContext design so operators can tune or disable startup page-fault prefetching per deployment instead of being stuck with the old fixed value.

Instructions:

1. The PR has to be tagged with at least one of the following labels (*):
   1. feature
   2. bugfix
   3. performance
   4. ui
   5. backward-incompat
   6. release-notes (**)
2. Remove these instructions before publishing the PR.

(*) Other labels to consider:

• testing
• dependencies
• docker
• kubernetes
• observability
• security
• code-style
• extension-point
• refactor
• cleanup

(**) Use release-notes label for scenarios like:

• New configuration options
• Deprecation of configurations
• Signature changes to public methods/interfaces
• New plugins added or old plugins removed

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.01%. Comparing base (bd1e882) to head (3285f23).

Files with missing lines	Patch %	Lines
.../starter/helix/HelixInstanceDataManagerConfig.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #19661      +/-   ##
============================================
+ Coverage     67.99%   68.01%   +0.01%     
  Complexity     1450     1450              
============================================
  Files          3511     3511              
  Lines        227483   227502      +19     
  Branches      35994    35995       +1     
============================================
+ Hits         154676   154729      +53     
+ Misses        60661    60640      -21     
+ Partials      12146    12133      -13     

Flag	Coverage Δ
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (?)
java-25	68.01% <90.90%> (+0.01%)	⬆️
lane-a	100.00% <ø> (ø)
lane-b	0.00% <ø> (ø)
temurin	68.01% <90.90%> (+0.01%)	⬆️
unittests	68.00% <90.90%> (+0.01%)	⬆️
unittests1	58.21% <95.00%> (+0.02%)	⬆️
unittests2	39.66% <59.09%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.