implement struct column assignment and validation logic

[guancioul]

Change Summary

This PR implements execution and validation logic for assigning values to nested struct fields in the OTAP columnar query engine, enabling queries such as:

logs | set resource.schema_url = "http://my-schema.com"
logs | set instrumentation_scope.name = "some_scope_name"

Validation (validate_assign in assign.rs)

• Add StructCol arm to the validate_assign dispatch
• Add is_valid_struct_field to check that the target field name actually exists in the struct schema
• Add type validation to reject assignments where the source value type does not match the destination field type
• Add validate_struct_col_assign_cardinality to enforce the OTAP 1:many hierarchy constraint (RESOURCE ==1:many==> SCOPE ==1:many==> LOG/SPAN/METRIC); e.g. set resource.schema_url = attributes["x"] is rejected because log-level data cannot be used to assign a resource-level field

Execution (execute in assign.rs)

• Add StructCol arm to the execution dispatch to handle struct field assignment
• Add assign_to_struct_column — handles non-null assignment to a struct field with row-level alignment between the source array and the target struct column
• Add assign_null_struct_field — handles null assignment by dropping the target field from the struct in the OTAP batch

DataScope Enhancement

• Extend DataScope with a new StructField variant to represent the cardinality level of a value read from a struct column field (e.g. reading resource.schema_url produces StructField(ResourceAttrs))
• Previously, reading a struct field like resource.schema_url as a source expression was treated as Root (log-level) scope — the same as any plain column — so the cardinality validator could not distinguish resource-level struct field reads from log-level reads
• With StructField, the validator can now correctly compare the cardinality level of the source expression (e.g. resource.schema_url → StructField(ResourceAttrs)) against the destination struct column (e.g. assigning to instrumentation_scope) and reject invalid assignments accordingly

What issue does this PR close?

• Closes [OTAP query-engine]: Support assignment of "struct" columns #2838

How are these changes tested?

• Assign a struct field from a static literal (e.g. resource.schema_url = "something")
• Assign a non-string (UInt32) struct field from a scalar literal (e.g. instrumentation_scope.dropped_attributes_count = 42)
• Reject assignment where source is at a lower cardinality level than the destination (e.g. resource.schema_url = attributes["x"])
• Allow assignment where source is at a higher or equal cardinality level (e.g. instrumentation_scope.name = resource.schema_url)
• Assign a struct field from its own sibling attributes (e.g. instrumentation_scope.name = instrumentation_scope.attributes["name"])
• Reject assignment of the wrong type (e.g. resource.dropped_attributes_count = "hello")
• Assign null to a struct field and verify the field is dropped from the batch
• Assign the result of a function expression (e.g. instrumentation_scope.name = substring(instrumentation_scope.name, 0, 6))
• Assign when the struct column does not yet exist in the batch
• Assign when the target field does not yet exist within an existing struct column

Are there any user-facing changes?

Yes. Users can now write OPL/KQL queries that assign values to nested struct fields such as resource.schema_url, instrumentation_scope.name, instrumentation_scope.version, and instrumentation_scope.dropped_attributes_count.

[codecov]

Codecov Report

❌ Patch coverage is 90.74316% with 71 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.93%. Comparing base (672d665) to head (b78f21d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2964      +/-   ##
==========================================
+ Coverage   85.92%   85.93%   +0.01%     
==========================================
  Files         725      725              
  Lines      275605   276347     +742     
==========================================
+ Hits       236811   237487     +676     
- Misses      38270    38336      +66     
  Partials      524      524              

Components	Coverage Δ
otap-dataflow	87.06% <90.74%> (+0.01%)	⬆️
query_abstraction	80.61% <ø> (ø)
query_engine	89.57% <ø> (ø)
otel-arrow-go	52.45% <ø> (ø)
quiver	92.25% <ø> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: guancioul / name: guancioul (7f63bb6, 941da4f)