RUM-18337: Fix slow frame continuity measurement and resend duration corrections in partial view updates

[hamorillo]

What does this PR do?

Two related fixes to RUM slow-frame tracking:

1. DefaultSlowFramesListener now measures the gap between consecutive slow frames from the end of the previous record (start + duration) instead of its start, so genuinely back-to-back slow frames are correctly merged into a single record instead of incorrectly starting a new one.
2. diffViewEvent's diffSlowFrames now resends the last known slow-frame record in a view update whenever its duration has changed, not just when a newer record with a later start appears — fixing a case where a growing record's duration correction was silently dropped from the diff after its first transmission.

Motivation

The two bugs compound each other: the first makes slow frames merge far more often once fixed, which increases exposure to the second.

Additional Notes

• No API surface changes.
• Also fixes a pre-existing test issue in DefaultSlowFramesListenerTest ("separate janks") where the listener was built with a hardcoded continuousSlowFrameThresholdNs = 16 instead of the test's own forged parameter.
• New regression tests added for both fixes; existing tests for the touched code paths pass unchanged.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• 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)

[datadog-datadog-prod-us1]

✅ All CI checks and tests passed. Datadog automation helped this PR pass.

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🔄 Datadog retried 1 test - 1 passed on retry

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 71.79% (+0.02%)

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