fix: peersync - case insensitive asset parsing

[ronballesteros]

Context

Related to #435

Nodes running the peersync-based daemon (post-v5.0.0) silently drop all inbound poll messages from peers still on v5.0.0, causing a one-way compatibility break.

NewAsset() performs a direct map lookup against stringToAsset, which only contains uppercase keys ("BTC", "LBTC"). v5.0.0 nodes advertise assets in lowercase in their poll payloads. The lookup fails, the poll is discarded, and observedAt never refreshes on the remote side. The remote peer ages the node off their active list within hours and cannot recover without restarting their daemon.

Logs on the node show:

  failed to parse poll message: invalid poll payload: unsupported asset: btc
  failed to parse poll message: invalid poll payload: unsupported asset: lbtc

Fix

Normalize the input with strings.ToUpper before lookup so both uppercase and lowercase symbols are accepted.

Testing

Verified manually against a v5.0.0 peer. Poll messages containing lowercase "btc"/"lbtc" are now parsed correctly and observedAt stays current.

[YusukeShimizu]

Approved. Thanks for the focused fix.
I also checked the previous misc_2 CI failure.
It was an unrelated integration-test/glightning shutdown race, and the rerun is green now.