Reject trailing characters in parseSize

[fallintoplace]

Summary

• validate the full suffix returned by stoull
• reject malformed values like 4kb, 1.5m, and 12junk
• add focused parser tests under src/perf/util, guarded by BUILD_TESTING

Test plan

• clang-format --dry-run --Werror
• strict Clang syntax checks for the parser and test source

[praktika-gh]

Workflow [PR], commit [4cd749d]

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Code Review

Result: ✅ No issues found

What changed: Updates parseSize to validate the entire suffix and reject malformed trailing characters. Adds focused unit tests and includes them only when BUILD_TESTING is enabled.

No issues found. The test target is now correctly guarded by BUILD_TESTING, and the parser validates the complete suffix as intended.

Investigation: 2/13 rounds, 11 tool calls.