fix: _set_status should not override existing span status (#3713)

[RiyaChaturvedi37]

Description

Fixes #3713

Problem

_set_status was unconditionally calling span.set_status(Status(status))
without checking if a status was already set. This caused two bugs:

• A previously set ERROR status could be downgraded to OK
• A previously set status description was wiped out (overridden with None)

Fix

Added a priority check before calling span.set_status:

• Never downgrade status (ERROR → OK or ERROR → UNSET are blocked)
• Preserve existing description if same status code and no new description provided

Tests

Added 4 new tests in test_semconv_status.py:

• test_does_not_downgrade_error_to_ok
• test_does_not_wipe_description_with_none
• test_upgrades_unset_to_error
• test_unset_to_ok

[MikeGoldsmith]

Looks good - thanks @RiyaChaturvedi37. I've left a suggestion for moving the dict out of the func and updating a test decription.

[MikeGoldsmith]

Thanks for the updates. There's still some more things to fix up before we can accept.

[RiyaChaturvedi37]

Hi @MikeGoldsmith, I've addressed all the feedback — moved the dict to module level, simplified the _set_span_status function, fixed the test description, and resolved the CHANGELOG conflict. Please let me know if there's anything else needed. Thank you!

[RiyaChaturvedi37]

Hi @MikeGoldsmith,
I've addressed all the feedback and wanted to give an update on the current state of the PR:
Regarding span.status suggestion:
I tried committing your suggestion to use span.status directly, but it caused 14 test failures in instrumentation-wsgi and instrumentation-asgi. The error was:
AttributeError: Mock object has no attribute 'status'
This happens because those tests use MagicMock(spec_set='Span'), which doesn't include status in its spec. The getattr(span, "status", None) form handles this safely by returning None instead of raising an AttributeError. I've reverted to getattr for this reason.
Regarding the check-links failure:
The only remaining CI failure is check-links, which contains pre-existing broken links not introduced by this PR. These include:
Malformed URLs with %5D encoding
Missing / in domain names
Deleted upstream specification files
This PR only modifies _semconv.py, test_semconv_status.py, and CHANGELOG.md — none of which contain these broken links.
All 750 other checks are now passing. Please let me know if any further changes are needed. Thank you!