core: Support struct field access in declarative calls

Add support for accessing struct fields in declarative calls using numeric
indices (e.g., event.params.asset.1). Previously, attempting to use struct
field access would fail with parsing errors.

Changes:
- Add EthereumArg::ParamField variant to support param.field patterns
- Update CallArg parser to handle 4-part patterns like event.params.asset.1
- Enhance address_for_log and args_for_log to extract struct fields
- Include declarative call labels in error messages for better debugging
- Add comprehensive tests for struct field access and error scenarios

Fixes #6059

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: fubhy