fix(proto): accept null AnyValue fields in OTLP JSON

[bruno-espino]

Changes

The ProtoJSON mapping requires parsers to accept null for any field and leave that field unset. The AnyValue protobuf definition permits its oneof to remain unset.

The custom AnyValue visitor introduced in #1753 deserializes recognized fields directly into their concrete type. As a result, {"intValue":null} fails with data did not match any variant of untagged enum StringOrInt instead of producing an empty AnyValue. This happens before the empty-value handling fixed in #3595, so it is a separate pre-existing case.

This occurs in real exporter traffic: the Vercel AI SDK can emit doubleValue: null for non-finite token attributes, and PostHog independently encountered whole-request failures in opentelemetry-proto and merged a receiver-side workaround that removes null-valued AnyValue fields before deserialization.

Deserialize each recognized field through Option, treating null as unset while preserving normal validation for non-null values. Regression coverage includes every AnyValue variant.

This was also tested through an OTLP receiver's JSON ingest path: upstream main rejected {"intValue":null} with the StringOrInt error; with this change it decodes as an unset value, while a fractional intValue remains rejected.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated
• Appropriate CHANGELOG.md updated
• No public API changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.2%. Comparing base (0e78170) to head (c21c95a).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3603   +/-   ##
=====================================
  Coverage   83.2%   83.2%           
=====================================
  Files        130     130           
  Lines      28246   28246           
=====================================
  Hits       23506   23506           
  Misses      4740    4740           

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[bruno-espino]

The integration test failure appears unrelated to this change, could a maintainer please rerun the failed job?