fix: default enum member value to id

[nanookclaw]

What changed

Enum member deserialization now accepts entries without an explicit value field and fills the public EnumEntriesSpec.value with the member id. Explicit value fields are still preserved.

This keeps downstream code working with a concrete ValueSpec while matching the semantic convention schema behavior described in #733.

Closes #733.

Verification

• cargo test -p weaver_semconv test_enum_member_value_defaults_to_id
• cargo test -p weaver_semconv

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.2%. Comparing base (f52329c) to head (7b13bfd).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1444   +/-   ##
=====================================
  Coverage   82.2%   82.2%           
=====================================
  Files        125     125           
  Lines      10241   10249    +8     
=====================================
+ Hits        8423    8431    +8     
  Misses      1818    1818           

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