[haphungw]

Convert BigQuery STRUCT columns into nested DescriptorProto messages in root.nested_type.

In BigQuery, a STRUCT column stores its subfields inline on TableFieldSchema.fields. In protobuf, a FieldDescriptorProto can't hold subfields inline: instead, the field sets type = TYPE_MESSAGE and type_name = "m1", and root.nested_type holds the DescriptorProto named "m1" with those subfields.

For (id INT64, address STRUCT<city STRING>), this produces the equivalent of:

message root {
  optional int64 id = 1;
  optional m1 address = 2;
  message m1 {
    optional string city = 1;
  }
}

Towards #6783

[gemini-code-assist]

Code Review

This pull request implements support for converting BigQuery STRUCT columns into nested Protobuf messages. It introduces recursive descriptor generation, handles nested error path propagation (e.g., a.b), avoids naming collisions with sibling columns, and adds comprehensive unit tests to validate these behaviors. I have no feedback to provide as the implementation is clean, robust, and conforms to the repository style guidelines.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.60%. Comparing base (bb112d0) to head (13ca964).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7463   +/-   ##
=======================================
  Coverage   98.60%   98.60%           
=======================================
  Files         338      338           
  Lines      148237   148319   +82     
=======================================
+ Hits       146172   146253   +81     
- Misses       2065     2066    +1     

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