(Calculated fields) Allow aggregations on calculated fields

[fulmicoton]

This plugs tantivy's jitexpr value sources for aggregations (quickwit-oss/tantivy#3162) into Quickwit.

SearchRequest gets a new calculated_fields field: a list of named jitexpr expressions, computed from fast fields. Aggregations can refer to these names like fast field names. This follows #6777, which added calculated-field predicates to the query AST.

"calculated_fields": [
  { "name": "#total", "expression": "(MULTIPLY price quantity)" }
],
"aggregation_request": "{\"by_color\": {\"terms\": {\"field\": \"color\"}, \"aggs\": {\"total\": {\"sum\": {\"field\": \"#total\"}}}}}"

Changes

• Bump tantivy to the tip of main, to get JitExprValueSourceProvider.
• New quickwit-search/src/calculated_fields.rs. It parses and type checks the calculated fields, and rejects empty and duplicate names.
• Leaf: when calculated_fields is not empty, the collector registers them in AggContextParams with with_value_sources. The merge collector does not need them, because merging intermediate results does not read columns.
• Warmup: the collector replaces the calculated fields used by aggregations with the fast fields their expressions read. Unused calculated fields are not warmed up.
• Root: invalid expressions are rejected before the fan-out. The inputs of a calculated field must be fast fields.
• Scroll requests drop calculated_fields, together with the aggregation.

A calculated field shadows a fast field with the same name, so a prefix such as # is recommended.

Not in this PR

• The REST search API and the Elasticsearch-compatible API do not expose calculated_fields yet. Only the proto/gRPC API does.
• Tantivy rejects some terms aggregation options on computed fields: _key ordering for text outputs, and min_doc_count: 0.

Tests

• Unit tests for parsing, the warmup resolution and the collector errors.
• test_single_node_aggregation_on_calculated_fields: a terms aggregation on (UPPER color) with a sum on (MULTIPLY price quantity).
• test_single_node_aggregation_calculated_field_requires_fast_inputs.

quickwit-doc-mapper's test_concatenate_multiple_field fails, but it fails on main too. It is not related to this PR.

🤖 Generated with Claude Code