[Fix] Make attribute encoding warnings exception-safe

[sbarrio]

What does this PR do?

The drop-warning paths in attribute encoding built their message with String(value), which itself throws on null-prototype objects or a throwing toString, so values that should have been dropped with a warning crashed the DdLogs/DdRum call. mapEncoder's catch handler had the same flaw, throwing again while reporting the first failure.

Adds safeToString (falling back to Object.prototype.toString.call) and uses it at those four sites, plus regression tests.

Motivation

Raised by #1426

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• Feature or bugfix MUST have appropriate tests
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