[RUM-13565] [V3] Fix incorrect encoding of internal attributes

[cdn34dd]

What does this PR do?

Prevent the flattening of nested attributes when handling internal attributes, so that native functionality remains unchanged.

Motivation

Incorrect handing of internal attributes:

const original = {
    '_dd.resource_timings': {
        firstByte: { duration: 56845703, startTime: 1767547597328000000 },
        download: { duration: 2200439, startTime: 1767547597384845800 },
        fetch: { duration: 59046143, startTime: 1767547597328000000 }
    }
};

const flattened = {
    '_dd.resource_timings.firstByte.duration': 56845703,
    '_dd.resource_timings.firstByte.startTime': 1767547597328000000,
    '_dd.resource_timings.download.duration': 2200439,
    '_dd.resource_timings.download.startTime': 1767547597384845800,
    '_dd.resource_timings.fetch.duration': 59046143,
    '_dd.resource_timings.fetch.startTime': 1767547597328000000
};

Correct V2 - https://mobile-integration.datadoghq.com/rum/sessions?query=%40type%3Asession %40application.id%3A056b6201-48cb-4acc-9fbc-48507b8e3e12&agg_m=count&agg_m_source=base&agg_t=count&event=AwAAAZtvNgjglADQwwAAABhBWnR2TmhLOUFBQXNpR0tzbE8ycHpBQUUAAAAkMDE5YjZmMzYtNWMzNS00ZDRmLWEwZGQtZThmY2FjODAxMzc3AAAAJg&fromUser=false&from_ts=1767028516231&to_ts=1767114916231&live=true

Incorrect V3 - https://mobile-integration.datadoghq.com/rum/sessions?query=%40type%3Asession %40application.id%3A056b6201-48cb-4acc-9fbc-48507b8e3e12&agg_m=count&agg_m_source=base&agg_t=count&event=AwAAAZtwObGPi4g5iAAAABhBWnR3T2I5TUFBREFZUVROTzQ1czhRQUQAAAAkZDE5YjcwMzktYjE4Zi00MDEzLWE4Y2QtMDI5N2E2OTZmNWFlAAAAAA&fromUser=false&from_ts=1767028752423&to_ts=1767115152423&live=true

Additional Notes

Anything else we should know when reviewing?

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)
• If this PR is auto-generated, please make sure also to manually update the code related to the change

[datadog-datadog-prod-us1]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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[sbarrio]

Very nice catch! 👏