What does this PR do?

Fixes RUM → APM correlation for resources tracked via DdUrlTransfer. RUM resource events now carry a backend-valid _dd.trace_id (32-char zero-padded hexadecimal) and _dd.span_id (decimal), so the rum-span-mapper backend can generate the corresponding APM span instead of silently dropping the event.

Trace IDs were appearing on the RUM side but the trace was never propagated to APM.

The rum-span-mapper backend validates each RUM resource before generating a span:

• _dd.span_id must be a strictly positive decimal number (NumberUtils.isDigits + > 0).
• _dd.trace_id must be either a decimal 64-bit number or a 128-bit hexadecimal padded to exactly 32 chars (ParsedTraceId.is128bitsHexEncodedTraceId).

The Roku SDK violated both:

A hex span_id containing any a–f fails isDigits, so for the W3C/B3 header types correlation essentially never worked; an unpadded trace_id (leading-zero nibbles) caused intermittent failures across all types.

Changes

• Added generateTraceId() / generateSpanId() helpers that generate an id once and expose every representation each consumer needs (decimal, 16-char and 32-char zero-padded hex).
• Reworked _addSampledInHeaders so the RUM event always stores traceId as 32-char padded hex and spanId as decimal, while each wire header keeps its own required format (datadog low-decimal + _dd.p.tid hex, b3/b3multi/tracecontext padded hex).

After change

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)