bugfix: Fix string code of `BLAKE2S_ADD_UINT256` hint #7106

Job	Run time
Build release binary for comparisons	2m 5s
Build the wasm demo	2m 31s
Build Cairo programs (cairo_bench_programs)	30s
Build Cairo programs (cairo_proof_programs)	21s
Build Cairo programs (cairo_test_programs)	24s
Build Cairo programs (cairo_1_test_contracts)	25s
Build Cairo programs (cairo_2_test_contracts)	19s
Run Lints	4m 30s
Compute memory and execution traces with cairo-lang (cairo_proof_programs, cairo_proof_trace, 1)	27s
Merge Cairo programs cache	16s
Compute memory and execution traces with cairo-lang (cairo_test_programs, cairo_trace, 2)	20s
Make sure all builds work	37m 42s
Run tests (test#1)	9m 29s
Run tests (test#2)	8m 27s
Run tests (test#3)	9m 27s
Run tests (test#4)	8m 37s
Run tests (test-no_std)	8m 28s
Run tests (test-wasm)	3m 34s
Run tests (lambdaworks-felt, test#1)	7m 43s
Run tests (lambdaworks-felt, test#2)	7m 52s
Run tests (lambdaworks-felt, test#3)	9m 36s
Run tests (lambdaworks-felt, test#4)	11m 26s
Run tests (lambdaworks-felt, test-no_std)	5m 30s
Run tests (lambdaworks-felt, test-wasm)	3m 21s
Compute memory and execution traces with cairo-vm (cairo_proof_programs, cairo_programs/proof_pro...	13s
Compute memory and execution traces with cairo-vm (cairo_test_programs, cairo_programs)	8s
Compare memory and execution traces from cairo-lang and cairo-vm (cairo_proof_programs)	24s
Compare memory and execution traces from cairo-lang and cairo-vm (cairo_test_programs)	19s
Upload coverage results to codecov.io	18s
	2h 24m 42s

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