app: add HeadBlockNotFullyVerified condition fallback BN

[KaloyanTanev]

An operator had an issue where the EL is not synced, but the CL is. However, Charon did not fallback to its fallback BNs.

{"log":"04:15:15.014 ERRO fetcher    Permanent failure calling fetcher/fetch: fetch attester data: beacon api attestation_data: nok http response: GET failed with status 500: {\"code\":500,\"message\":\"UNHANDLED_ERROR: HeadBlockNotFullyVerified { beacon_block_root: 0x296d87292e7533cce4abfb1cd83e96bb5692d7f96498398109ccc653d125fc46, execution_status: Optimistic(0x8fc7a74d22d5272c2b938db92bbd7866a08cb4579d1523b26c281281eb5869d8) }\",\"stacktraces\":[]} {\"label\": \"attestation_data\", \"status_code\": 500, \"endpoint\": \"/eth/v1/validator/attestation_data\", \"method\": \"GET\", \"data\": \"{\\\"code\\\":500,\\\"message\\\":\\\"UNHANDLED_ERROR: HeadBlockNotFullyVerified { beacon_block_root: 0x296d87292e7533cce4abfb1cd83e96bb5692d7f96498398109ccc653d125fc46, execution_status: Optimistic(0x8fc7a74d22d5272c2b938db92bbd7866a08cb4579d1523b26c281281eb5869d8) }\\\",\\\"stacktraces\\\":[]}\", \"duty\": \"13728074/attester\"}\n","stream":"stderr","time":"2026-02-20T04:15:15.014464971Z"}

category: feature
ticket: none

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.43%. Comparing base (4070c56) to head (ab49fd5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4332      +/-   ##
==========================================
+ Coverage   56.39%   56.43%   +0.03%     
==========================================
  Files         237      237              
  Lines       31660    31660              
==========================================
+ Hits        17856    17866      +10     
+ Misses      11517    11506      -11     
- Partials     2287     2288       +1     

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