Handle malformed RPN calculator input safely

[krotname]

Motivation

• The CLI Calculator previously assumed well-formed RPN tokens and performed unchecked stack pops, division, and integer parsing which allowed malformed stdin (e.g. +, empty input, 1 0 /, abc, or extra operands) to throw runtime exceptions and terminate the process. The change prevents these crash/availability paths by validating untrusted input.

Description

• Validate operand counts before applying an operator by checking st.size() < 2 and reject with a controlled error instead of performing unchecked pop() calls in eval (src/main/java/algorithms/sprint2/Calculator.java).
• Detect division-by-zero before calling Math.floorDiv and reject with a clear error message.
• Convert invalid numeric tokens by wrapping parseInt in a try/catch and rethrowing as IllegalArgumentException, and require the final stack size to be exactly one before returning the result.
• Handle IllegalArgumentException at the CLI boundary in run() by printing a short diagnostic to stderr and returning instead of allowing stack/parse/arithmetic exceptions to propagate.
• Add regression coverage for malformed inputs by adding a parameterized test to src/test/java/algorithms/AlgorithmCliTest.java that verifies the Calculator rejects +, empty input, 1 0 /, abc, and an extra-operand case.

Testing

• Ran mvn -q -Dtest=algorithms.AlgorithmCliTest test and the test suite for the modified integration case completed successfully.
• Ran mvn -q test to execute the full test suite and it completed without failures.
• Ran mvn -q checkstyle:check and code passes the project's checkstyle rules.
• Ran git diff --check to ensure no whitespace or patch issues and it reported clean results.

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Codex Task

[krotname]

Superseded by the validated combined merge in #115: #115