RFR: JDK-8374395 : Improve MemoryUsage.toString() and constructor error messages for better clarity

[eunbin son]

### Summary
MemoryUsage.toString() now displays "N/A" for undefined init and max values (-1) instead of "-1(-1K)".

This addresses the first of three issues in JDK-8374395.

### Description
When init or max is -1 (undefined), toString() outputs "-1(-1K)". The Javadoc states that -1 means undefined, but the output does not clearly indicate this.

toString() checks if init or max equals -1 and outputs "init = N/A" or "max = N/A" respectively. Output format for valid values remains unchanged.

Added test cases to the existing jtreg test file. All tests pass.

Bug-ID : JDK-8374395 
https://bugs.java.com/bugdatabase/view_bug?bug_id=JDK-8374395

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Commit messages:
 - Update full name
 - 8374395: Clarify undefined values in MemoryUsage.toString()

Changes: https://git.openjdk.org/jdk/pull/29009/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29009&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374395
  Stats: 100 lines in 2 files changed: 96 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/29009.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29009/head:pull/29009

PR: https://git.openjdk.org/jdk/pull/29009