fix: handle legacy string-encoded customHeaders in ConnectionOptionsOAuth2

[fern-api]

🔧 Changes

ConnectionOptionsOAuth2.UnmarshalJSON fails with json: cannot unmarshal string into Go struct field ... of type map[string]string when the Management API returns customHeaders as a legacy JSON-encoded string instead of a map object, which crashes terraform-provider-auth0 on affected tenants.

This change makes the unmarshaler tolerant of the legacy format:

• "customHeaders": {"key": "value"} (current format) — decoded into *ConnectionCustomHeadersOAuth2 as before.
• "customHeaders": "<json-encoded-string>" (legacy format) — the field is left unset (nil) instead of returning an error, since the legacy value cannot be reliably decoded.
• Any other non-null, non-string, non-object value still returns an unmarshal error.

Marshaling behavior is unchanged.

📚 References

Fixes #815

🔬 Testing

Added TestConnectionOptionsOAuth2UnmarshalCustomHeaders covering the map format, legacy string format, null, absent field, invalid type, and marshal round-trip. The legacy string case fails without this change with the exact error from #815 and passes with it. Full go test ./management/ passes.

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.90%. Comparing base (ea264e2) to head (d128ad0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #817   +/-   ##
=======================================
  Coverage   86.90%   86.90%           
=======================================
  Files         387      387           
  Lines      162844   162852    +8     
=======================================
+ Hits       141526   141534    +8     
  Misses      15990    15990           
  Partials     5328     5328           

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