Use fixed-string matching in sudoku-solver tests

[Copilot]

grep -q treats the pattern as a regex, which can silently misfire on special characters. The expected solution string and "Found 1 solution(s)." (contains ( and )) should be matched literally.

Changes

• sudoku-solver/tests.sh: Replace grep -q with grep -Fq on both assertion lines to enforce fixed-string matching

-if ! echo "$output" | grep -q "$expected"; then
+if ! echo "$output" | grep -Fq "$expected"; then
 
-if ! echo "$limited_output" | grep -q "Found 1 solution(s)."; then
+if ! echo "$limited_output" | grep -Fq "Found 1 solution(s)."; then

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