v3.0: fix: enable on-demand leader schedule computation in get_slot_leaders (backport of #7765)

[mergify]

Fix get_slot_leaders epoch transition failures

Problem

get_slot_leaders RPC was failing with "Invalid slot range: leader schedule for epoch X is unavailable" for approximately 31 slots during every epoch transition. This occurred because:

• get_slot_leaders only checked leader_schedule_cache.get_epoch_leader_schedule(epoch)
• During epoch transitions, the new epoch's leader schedule isn't cached until the first slot of that epoch is rooted
• This created a ~31 slot window where valid requests would fail
• Caused network spam as clients sent transactions to wrong leaders during transitions

Solution

• Add fallback to leader_schedule_utils::leader_schedule() on cache miss
• Enables on-demand computation when bank has stake information available
• Preserves all existing behavior and error handling
• No performance impact for cached schedules

Closes #6845 , can you pls check. @KirillLykov

---

This is an automatic backport of pull request #7765 done by [Mergify](https://mergify.com).

[mergify]

If this PR represents a change to the public RPC API:

1. Make sure it includes a complementary update to rpc-client/ (example)
2. Open a follow-up PR to update the JavaScript client @solana/kit (example)

Thank you for keeping the RPC clients in sync with the server API @mergify[bot].

[gregcusack]

lgtm

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.4%. Comparing base (ddca2f9) to head (16ba910).

Additional details and impacted files

@@           Coverage Diff           @@
##             v3.0    #7856   +/-   ##
=======================================
  Coverage    83.4%    83.4%           
=======================================
  Files         814      814           
  Lines      366089   366117   +28     
=======================================
+ Hits       305517   305567   +50     
+ Misses      60572    60550   -22     

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[t-nelson]

i dunno that this was the correct fix. commented on master pr #7765 (comment)

[t-nelson]

closing since master pr is slated for revert #7917