fix: limit FVM concurrency + use blocking tasks

[LesnyRumcajs]

Summary of changes

Changes introduced in this pull request:

• this change reduces the number of concurrent FVM invocations which seem to degrade results

Reference issue to close (if applicable)

Closes

Other information and links

Before

=== oha ===
--- forest (http://127.0.0.1:2345/rpc/v1) ---
warmup (200 requests)...
Summary:
  Success rate:	100.00%
  Total:	31.2324 sec
  Slowest:	1.2056 sec
  Fastest:	0.0816 sec
  Average:	0.3112 sec
  Requests/sec:	320.1802

  Total data:	6.51 MiB
  Size/request:	683 B
  Size/sec:	213.56 KiB

Response time histogram:
  0.082 sec [1]    |
  0.194 sec [2001] |■■■■■■■■■■■■■■■■■
  0.306 sec [3595] |■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■
  0.419 sec [2666] |■■■■■■■■■■■■■■■■■■■■■■■
  0.531 sec [1061] |■■■■■■■■■
  0.644 sec [403]  |■■■
  0.756 sec [149]  |■
  0.868 sec [68]   |
  0.981 sec [35]   |
  1.093 sec [15]   |
  1.206 sec [6]    |

Response time distribution:
  10.00% in 0.1665 sec
  25.00% in 0.2039 sec
  50.00% in 0.2896 sec
  75.00% in 0.3819 sec
  90.00% in 0.4828 sec
  95.00% in 0.5683 sec
  99.00% in 0.7868 sec
  99.90% in 1.0704 sec
  99.99% in 1.1716 sec


Details (average, fastest, slowest):
  DNS+dialup:	0.0001 sec, 0.0000 sec, 0.0003 sec
  DNS-lookup:	0.0000 sec, 0.0000 sec, 0.0000 sec

Status code distribution:
  [200] 10000 responses

After

=== oha ===
--- forest (http://127.0.0.1:2345/rpc/v1) ---
warmup (200 requests)...
Summary:
  Success rate:	100.00%
  Total:	31713.7928 ms
  Slowest:	566.0080 ms
  Fastest:	170.1453 ms
  Average:	316.3254 ms
  Requests/sec:	315.3202

  Total data:	6.51 MiB
  Size/request:	683 B
  Size/sec:	210.32 KiB

Response time histogram:
  170.145 ms [1]    |
  209.732 ms [35]   |
  249.318 ms [27]   |
  288.904 ms [82]   |
  328.490 ms [9018] |■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■
  368.077 ms [796]  |■■
  407.663 ms [17]   |
  447.249 ms [8]    |
  486.835 ms [9]    |
  526.422 ms [2]    |
  566.008 ms [5]    |

Response time distribution:
  10.00% in 305.8730 ms
  25.00% in 311.0561 ms
  50.00% in 316.5614 ms
  75.00% in 322.0646 ms
  90.00% in 327.5615 ms
  95.00% in 331.1979 ms
  99.00% in 345.9131 ms
  99.90% in 462.0490 ms
  99.99% in 561.3125 ms


Details (average, fastest, slowest):
  DNS+dialup:	0.1221 ms, 0.0367 ms, 0.3415 ms
  DNS-lookup:	0.0032 ms, 0.0004 ms, 0.0268 ms

Status code distribution:
  [200] 10000 responses

results written to results/eth_estimateGas-20260522T112614Z.*

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

Outside contributions

• I have read and agree to the CONTRIBUTING document.
• I have read and agree to the AI Policy document. I understand that failure to comply with the guidelines will lead to rejection of the pull request.

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