SQL Expressions: field metadata discarded (DisplayNameFromDS, Filterable) causing display and interaction regressions

## Description When data passes through SQL Expressions, field metadata (`FieldConfig`) is discarded. This causes user-visible regressions where: 1. Human-readable column names revert to internal field names 2. Click-to-filter functionality for ad-hoc filters is lost Even a no-op query like `SELECT * FROM A` causes these regressions. ## Root Cause The SQL engine creates new fields for query output without preserving the input fields' `Config`: **Input conversion** (`pkg/expr/sql/frame_table.go:SchemaFromFrame`): ```go schema[i] = &mysql.Column{ Name: field.Name, // copied Type: convertDataType(...), // field.Config is NOT preserved } ``` **Output conversion** (`pkg/expr/sql/frame_db_conv.go:convertToDataFrame`): ```go field := data.NewFieldFromFieldType(fT, 0) field.Name = col.Name // field.Config is nil - metadata lost ``` ## Affected Metadata | Property | Purpose | Impact when lost | |----------|---------|------------------| | `DisplayNameFromDS` | Human-readable field/column name | Users see internal names like `Account.Owner.Name` instead of `Account Owner Name` | | `Filterable` | Enables click-to-filter for ad-hoc filters | Users can no longer click values in tables/charts to add dashboard filters | ## Affected Datasources **Salesforce** - Sets `DisplayNameFromDS` on all columns: ```go // pkg/plugin/schema.go config := &data.FieldConfig{ DisplayNameFromDS: alias, // Human-readable column name } ``` **Elasticsearch** - Sets `Filterable` on log/raw data fields: ```go // pkg/tsdb/elasticsearch/dataframe_builder.go isFilterable := true field.Config = &data.FieldConfig{Filterable: &isFilterable} ``` **Loki** - Sets `Filterable` on label columns in instant metric tables: ```typescript // public/app/plugins/datasource/loki/makeTableFrames.ts const labelFields = sortedLabelNames.map((labelName) => ({ name: labelName, config: { filterable: true }, ... })); ``` **Prometheus** - Similar patterns for instant query results. ## Reprod…