perf(rpc): avoid IndexedBatch allocation in read-only state views

[thiagodeev]

Read-only state views (HeadState, StateAtBlockNumber, StateAtBlockHash) in deprecatedStateBackend previously allocated a Pebble IndexedBatch per call, paying the batch setup cost and the batch-overlay merge on every subsequent Get - despite never writing. They now read the database directly through a readOnlyTxn adapter that satisfies the db.IndexedBatch interface deprecatedstate.New requires but rejects writes.

Also adds BenchmarkNonce, benchmarking Handler.Nonce against a real Pebble-backed state with 50k contracts under pseudorandom addresses.

Numbers

BenchmarkNonce, benchstat over 6 runs each (-benchtime=2s):

metric	main	this PR	delta
time/op	4.184us +-7%	3.392us +-4%	-18.92%
B/op	2031	374	-81.59%
allocs/op	19	12	-36.84%

Every read-only RPC that goes through these state views benefits, not just starknet_getNonce.

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.51%. Comparing base (d5d68c0) to head (0d237a9).

Files with missing lines	Patch %	Lines
blockchain/statebackend/deprecated.go	50.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3941      +/-   ##
==========================================
- Coverage   74.63%   74.51%   -0.13%     
==========================================
  Files         462      462              
  Lines       40989    40990       +1     
==========================================
- Hits        30592    30543      -49     
- Misses       8294     8347      +53     
+ Partials     2103     2100       -3     

Flag	Coverage Δ
jsonv2	77.36% <100.00%> (+0.01%)	⬆️

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