Deflake async peer registration and harden PeerList.All

[masih]

TestReactor_BlockProviders flakes because peer connectivity is established before the reactor has asynchronously processed peer-updates and appended all peers to r.peers.

Fix this categorically by:

• waiting in reactorTestSuite.AddPeer until the new peer is visible in reactor.peers before returning;
• making PeerList.All() return a copy (snapshot) instead of the internal slice;
• adding TestPeerListAllReturnsSnapshot to enforce snapshot semantics.

This removes timing-sensitive reads of partially-registered peers and prevents callers from observing/mutating shared peer-list backing storage.

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Mar 2, 2026, 3:34 PM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.23%. Comparing base (b6685c1) to head (3613fd5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2999   +/-   ##
=======================================
  Coverage   58.22%   58.23%           
=======================================
  Files        2113     2113           
  Lines      173528   173530    +2     
=======================================
+ Hits       101038   101051   +13     
+ Misses      63483    63474    -9     
+ Partials     9007     9005    -2     

Flag	Coverage Δ
sei-chain-pr	73.16% <100.00%> (?)
sei-db	70.41% <ø> (ø)

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

Files with missing lines	Coverage Δ
sei-tendermint/internal/statesync/dispatcher.go	79.68% <100.00%> (+5.87%)	⬆️

... and 2 files with indirect coverage changes

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