Rename ZIPCompressedMessage getter to getCompressionLevel and keep getLevel as deprecated bridge

[Copilot]

This stacked PR addresses review feedback on #2783 to make the ZIP compression API clearer without breaking callers. The getter is renamed to getCompressionLevel, while getLevel is retained as a deprecated compatibility path.

• API update (ZIPCompressedMessage)

  • Introduced getCompressionLevel() as the primary instance getter.
  • Kept getLevel() as @Deprecated, delegating directly to getCompressionLevel().
  • Added Javadoc on getLevel() explicitly directing users to getCompressionLevel().

• Internal call-site migration

  • Updated ZIPSerializer to use zipMessage.getCompressionLevel().

• Test alignment + compatibility coverage

  • Updated existing assertions to call getCompressionLevel().
  • Added a focused test ensuring deprecated getLevel() returns the same value via delegation.

public int getCompressionLevel() {
    return compressionLevel;
}

/** @deprecated Use {@link #getCompressionLevel()}. */
@Deprecated
public int getLevel() {
    return getCompressionLevel();
}

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

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• dl.google.com
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If you need me to access, download, or install something from one of these locations, you can either:

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