feat: dd-trace 5.0 to 6.0 - replace experimental b3 config

[mohab-sameh]

Summary

Handle removed experimental.b3 and DD_TRACE_EXPERIMENTAL_B3_ENABLED usage.

Detection Criteria

Detect:

• experimental: { b3: <boolean> } in .init(...) options
• DD_TRACE_EXPERIMENTAL_B3_ENABLED in source/config files

Transformation Logic

Use a conservative plan:

• Current implementation reports remaining experimental.b3 and
  DD_TRACE_EXPERIMENTAL_B3_ENABLED through the optional final AI review in
  codemods/dd-trace-5-to-6-migration-recipe.
• Future deterministic rewrite can add DD_TRACE_PROPAGATION_STYLE or
  top-level propagationStyle only when no propagation-style config exists.
• If disabled or if propagation-style config already exists, keep it manual.

Before / After Example

tracer.init({
  experimental: { b3: true }
});

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

Notes / Edge Cases

Needs final confirmation of exact experimental.b3 semantics before broad
rewrite. Existing inject/extract propagation settings must not be overwritten.