libbeat/common: make Round results identical across architectures

[AndersonQ]

Proposed commit message

    libbeat/common: make Round results identical across architectures
    
    Round scales a value and then rounds it. The rounding step relied on
    floating-point behaviour that is not guaranteed to be identical across CPU
    architectures, so for values sitting exactly on a rounding midpoint (for
    example 0.50005) amd64 and arm64 could disagree: the same input produced a
    different rounded number depending on which architecture the beat ran on.
    This surfaced as the libbeat arm64 unit tests failing after the Go 1.26
    upgrade.
    
    Rounding now uses the standard library's math.Round on the scaled value, a
    well-defined operation that every architecture computes the same way. Results
    are now stable and portable regardless of CPU or compiler, and the behaviour
    the tests pin down is unchanged.
    
    Assisted-By: Claude Code

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability. - the existing tests are now passing
• [ ] I have added an entry in ./changelog/fragments using the changelog tool.

Disruptive User Impact

None

How to test this PR locally

run on ARM:

go test -v -run TestRound ./libbeat/common/

Related issues

• N/A

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[Copilot]

Pull request overview

This PR updates libbeat/common.Round to make rounding deterministic across CPU architectures by using math.Round on the scaled value, avoiding architecture-dependent edge behavior at midpoint boundaries (e.g. 0.50005 at 4 decimal places).

Changes:

• Replace custom midpoint rounding logic (Modf + Ceil/Floor) with math.Round on the scaled value.
• Update the Round function comment to explicitly describe half-away-from-zero behavior.

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[andrzej-stencel]

This change mirrors https://github.com/elastic/elastic-agent-system-metrics/pull/295/changes#diff-5ecedbe075db3830efdb44b74505a8748573ed54c5679946f377d3a7db1adf70 👍

I'd suggest to add tests for negative values, as it is changing with this change.

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