v2.3: 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].

[KirillLykov]

To address why I think it worth backporting -- because this long existing bug breaks leader targeting for clients using ConnectionCache (or tpu-client-next with default leader targeting) after the epoch change.

[KirillLykov]

lets backport it only on 3.0