fix: preserve metadata key casing in multipart form-data requests

[tasdrake]

When sending messages with a total payload >= 3MB, the SDK switches from JSON to multipart/form-data serialization. The form-data path in _buildFormRequest() was calling objKeysToSnakeCase() without excluding the metadata field, causing metadata keys like myCustomKey to be silently converted to my_custom_key. The JSON path in apiClient.ts already correctly excluded metadata from case conversion.

This fix adds the ['metadata'] exclusion to both _buildFormRequest() and calculateTotalPayloadSize(), making metadata handling consistent across all serialization paths (messages, drafts create/update).

Includes a regression test that verifies camelCase metadata keys survive the multipart serialization path unchanged.

License

I confirm that this contribution is made under the terms of the MIT license and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.72%. Comparing base (ca893af) to head (cd08e04).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #729   +/-   ##
=======================================
  Coverage   98.72%   98.72%           
=======================================
  Files          39       39           
  Lines         860      860           
  Branches       69       69           
=======================================
  Hits          849      849           
  Misses         11       11           

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