feat(preconf): add observability metrics for sequencer and validator flows

[fridrik01]

This PR adds observability metrics for preconf sequencer/validator flows:

• Sequencer-side: request outcomes, proposer-check results
• Validator-side: fetch outcomes, fallback reasons, sequencer-available gauge
• Blockchain: round-change counter
• Internal errors now return 500 (was 404) so HTTP status matches the metric label

[codecov]

Codecov Report

❌ Patch coverage is 37.03704% with 51 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.83%. Comparing base (cd11467) to head (6cd2b87).

Files with missing lines	Patch %	Lines
beacon/validator/block_builder.go	0.00%	28 Missing ⚠️
beacon/validator/preconf_metrics.go	16.66%	10 Missing ⚠️
beacon/preconf/server.go	79.16%	5 Missing ⚠️
beacon/validator/service.go	50.00%	2 Missing and 1 partial ⚠️
beacon/blockchain/metrics.go	0.00%	2 Missing ⚠️
beacon/blockchain/round_change.go	0.00%	2 Missing ⚠️
node-core/components/preconf_server.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           preconf-dev    #3084      +/-   ##
===============================================
- Coverage        58.93%   58.83%   -0.10%     
===============================================
  Files              379      381       +2     
  Lines            19531    19603      +72     
===============================================
+ Hits             11511    11534      +23     
- Misses            7058     7106      +48     
- Partials           962      963       +1     

Files with missing lines	Coverage Δ
beacon/preconf/server_metrics.go	100.00% <100.00%> (ø)
node-core/components/preconf_server.go	22.22% <0.00%> (-0.64%)	⬇️
beacon/blockchain/metrics.go	81.39% <0.00%> (-3.98%)	⬇️
beacon/blockchain/round_change.go	0.00% <0.00%> (ø)
beacon/validator/service.go	89.65% <50.00%> (-10.35%)	⬇️
beacon/preconf/server.go	52.57% <79.16%> (+2.29%)	⬆️
beacon/validator/preconf_metrics.go	16.66% <16.66%> (ø)
beacon/validator/block_builder.go	48.61% <0.00%> (-4.08%)	⬇️

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[calbera]

nit: could make this a typed string type ServerResult string and just have the value constants be OK, Unauthorized, etc..

fetchAndWritePayload would then return ServerResult