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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[LesnyRumcajs]

Superseded by #7184