Preserve OpenCensus status description when converting to OpenTelemetry

[thswlsqls]

Fixes #8508

Description

• OpenTelemetrySpanImpl.setStatus(io.opencensus.trace.Status) mapped only the status code (OK/ERROR) and dropped the OpenCensus status description, so Status.UNKNOWN.withDescription("connection refused") reached exporters as an ERROR with no description.
• Now read status.getDescription() and forward it through the existing setStatus(StatusCode, String) delegation — the sibling overload that already carries a description to the OpenTelemetry span — falling back to the code-only overload when the description is null.
• No description branch is needed for OK: SdkSpan.setStatus ignores a description when the status is not ERROR.

Testing done

• Added OpenTelemetrySpanImplTest#setStatus_preservesDescription (description forwarded as setStatus(ERROR, "...")) and #setStatus_withoutDescription_setsCodeOnly (no description delegates the code-only overload).
• ./gradlew :opencensus-shim:check — passed (new test class: 2 tests).
• Added a ## Unreleased entry in CHANGELOG.md (status description is now visible in exported span status).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.77%. Comparing base (824334c) to head (6a91dc3).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8511   +/-   ##
=========================================
  Coverage     78.77%   78.77%           
- Complexity     8579     8580    +1     
=========================================
  Files          1009     1009           
  Lines         28993    28997    +4     
  Branches       3599     3600    +1     
=========================================
+ Hits          22839    22843    +4     
  Misses         5311     5311           
  Partials        843      843           

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