Simplify ChannelUnavailable APIError handling with let-else

[Abeeujah]

e9e3060 Fixes the TODO for the error handling.
3d6a879 Fixes the triggered manual_str_repeat lint.

[ldk-reviews-bot]

I've assigned @valentinewallace as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[ldk-claude-review-bot]

No issues found.

Both changes are correct and behavior-preserving:

• channelmanager.rs:11468-11473 — the let-else refactor is equivalent to the original match. handle_error always returns Err for an Err input (it's a map_err), so .unwrap_err() cannot panic, and res remains owned/usable in the else block since a failed match moves nothing.
• macro_logger.rs — "a".repeat(n) / "\u{1F600}".repeat(n) are semantically identical to the previous iter::repeat(c).take(n).collect(), including the multibyte case.

[Hamza1610]

Test Results: PASS

All tests passed successfully! However, there are a couple of minor warnings to address:

** Redundant Import in channelmanager.rs**
warning: the item Hasher is imported redundantly
--> lightning/src/ln/channelmanager.rs:19832:35
|
177 | use core::hash::{Hash, Hasher};
| ------ the item Hasher is already imported here
19832 | use core::hash::{BuildHasher, Hasher};
| ^^^^^^

Issue: Hasher is imported twice in the same file. Fix: Remove the redundant Hasher from line 19832, keeping only BuildHasher: use core::hash::{BuildHasher};

Overall: Tests pass |Only minor cleanup needed

[Hamza1610]

When I was testing I noticed some ignored test. These might be deprecated test that are not in used anymore. I'm suggesting to create a new PR for this. @casey what do you think about that.

[Abeeujah]

use core::hash::{Hash, Hasher};

Hi @Hamza1610

I appreciate the review! However, the unused import warning is actually unrelated to the changes in this PR. It looks like it was introduced to main in commit a1ad1a3

Given the ongoing Forgejo migration (#4734), it's probably best to avoid scope creep here to keep this PR clean and prevent untracked changes.