parseBuildOutputTimestamp rejects negative epoch timestamps with misleading error

[elharo]

Affected version: 3.6.0+

Description:

parseBuildOutputTimestamp() at MavenArchiver.java:690 uses isNumeric() to detect integer epoch timestamps, but isNumeric() rejects the minus sign:

for (char c : str.toCharArray()) {
    if (!Character.isDigit(c)) {
        return false;
    }
}

So isNumeric(-1) returns false. Execution falls through to the ISO-8601 parser at line 709, which also fails, throwing:

IllegalArgumentException("Invalid project.build.outputTimestamp value '-1'")

Negative epoch timestamps (seconds before 1970-01-01) are valid per POSIX and could reasonably appear in SOURCE_DATE_EPOCH or timestamp configuration.

Impact: Valid negative timestamps are rejected with a confusing error message.

Fix: Allow an optional leading - in the numeric check.

[elharo]

There are no actual Maven builds before 1970 (or 1990 for that matter). Maybe we want the exception? or a better exception? Though if this is intentional we need to document it.