Feat: Have SpanContext support the new W3C random flag.

[nikhilmantri0902]

Problem

• The Go SDK masks trace flags to the sampled bit and rejects version 00 flags > 0x02.
• This drops the W3C Trace Context Level 2 random Trace ID flag (0x02) and any forward-compatible bits.

Approach

• Treat trace flags as an 8-bit bitmask across extract, storage, and inject.
• Accept any flags for traceparent version 00 and preserve them unchanged.
• Interpret only known bits for behavior (sampling uses 0x01); do not change sampling logic.

Implementation details

• propagation/trace_context.go
  • Inject: emit sc.TraceFlags() without masking.
  • Extract: remove opts[0] > 2 rejection; store full flags byte in SpanContextConfig.TraceFlags.
• propagation/trace_context_test.go
  • Add extract cases for 0x02 and 0x03; ensure 0x09 is preserved.
  • Remove "unused bits set" from invalid cases.
  • Inject cases preserve 0xff, 0x02, and 0x03.
  • Future-version flag tests now expect full byte, not masked.

Closes #7635

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.6%. Comparing base (edba765) to head (47af92a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7834   +/-   ##
=====================================
  Coverage   81.6%   81.6%           
=====================================
  Files        304     304           
  Lines      23384   23385    +1     
=====================================
+ Hits       19090   19091    +1     
+ Misses      3906    3905    -1     
- Partials     388     389    +1     

Files with missing lines	Coverage Δ
propagation/trace_context.go	97.0% <100.0%> (+<0.1%)	⬆️
trace/trace.go	98.2% <ø> (ø)

... and 4 files with indirect coverage changes

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[nikhilmantri0902]

Hi @dmathieu can you review this.

[nikhilmantri0902]

accepted ur suggested change @dmathieu. We can merge this I think now.