Harden AverageElement against malformed and oversized input

[krotname]

Motivation

• The CLI solver accepted a single unbounded stdin line and immediately split and parsed every token with Integer.valueOf, which allowed EOF, blank, non-numeric tokens, too-few tokens, and extremely long lines to crash or exhaust resources.
• The previous implementation materialized and sorted the entire input before accessing the second element, enabling memory/CPU exhaustion on attacker-controlled input.
• The change aims to make input handling robust and to compute the required result without unbounded allocation while preserving the original semantics for valid inputs.

Description

• Bound stdin processing by adding MAX_INPUT_CHARACTERS = 4096 and a readBoundedLine(Reader) helper that returns null for EOF, blank, or oversized lines.
• Change solve(Reader, Writer) to use the bounded reader, normalize whitespace with split("\\s+"), and silently ignore invalid, blank, undersized, or oversized input by returning early.
• Replace stream-based parsing + sorting + toList().get(1) with a linear, constant-space algorithm that scans parsed integers once to compute the second-smallest value and throws IllegalArgumentException for fewer than two values.
• Add regression tests in src/test/java/coderun/AverageElementTest.java covering empty/blank input, single-value input, non-numeric tokens, oversized input, duplicates, and multi-value behavior, and keep whitespace-normalization test.

Testing

• Ran the specific unit tests with mvn -q -Dtest=coderun.AverageElementTest test and they passed.
• Ran the full test suite with mvn -q test and it completed successfully.
• Verified no git diff --check issues were reported after the changes.

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Codex Task

[krotname]

Superseded by the validated combined merge in #115: #115