feat: dd-trace 5.0 to 6.0 - rename b3 single header propagation style

[mohab-sameh]

Summary

Rename the legacy propagation style string "b3 single header" to canonical
"b3".

Detection Criteria

Detect exact string values in propagation-style contexts:

• propagationStyle
• DD_TRACE_PROPAGATION_STYLE
• assignments to process.env.DD_TRACE_PROPAGATION_STYLE

Transformation Logic

Replace the exact value with "b3".

Before / After Example

tracer.init({ propagationStyle: ["datadog", "b3 single header"] });

tracer.init({ propagationStyle: ["datadog", "b3"] });

Notes / Edge Cases

Do not rewrite unrelated string literals. Programmatic propagationStyle
rewrites must be inside a dd-trace .init(...) call. The package workflow
also has a gated AI config step for exact active propagation-style config in
.env, YAML, Dockerfile, shell, CI, and deployment files.