feat: add StockSpanProblem algorithm

[oleksii-tumanov]

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Description

Adds a monotonic-stack implementation of the Stock Span Problem.

The implementation:

• computes the span for each day in linear time
• returns an array of spans for the input prices
• validates null input

Closes #7311

Test Case

Validated with:

• mixed prices: [10, 4, 5, 90, 120, 80] -> [1, 1, 2, 4, 5, 1]
• increasing prices
• decreasing prices
• repeated equal prices
• empty array
• single-element array
• null input throws IllegalArgumentException

Commands used:

• mvn -Dtest=StockSpanProblemTest test
• mvn -DskipTests checkstyle:check

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.45%. Comparing base (5e06b15) to head (105aae5).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #7312   +/-   ##
=========================================
  Coverage     79.44%   79.45%           
- Complexity     7064     7070    +6     
=========================================
  Files           788      789    +1     
  Lines         23129    23139   +10     
  Branches       4548     4551    +3     
=========================================
+ Hits          18375    18385   +10     
  Misses         4022     4022           
  Partials        732      732           

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