feat: add BuildJsonApiQueryAsync for custom query execution

[Erlend Ellefsen (erlendellefsen)]

• Add BuildJsonApiQueryAsync<T>() method to JsonApiController
• Returns processed IQueryable without pagination for exports/aggregations
• Add JsonApiQueryResult<T> result class with Query, Parameters, TotalCount
• Add 17 tests covering filters, includes, sorting, count behavior
• Consolidate statistics docs into new build-query.md

[Copilot]

Pull request overview

This PR adds a new BuildJsonApiQueryAsync method to enable custom query execution scenarios like CSV exports, aggregations, and projections. The method processes JSON:API query parameters (filters, includes, sorting) but intentionally does not apply pagination, allowing developers to handle the final query execution according to their needs.

Changes:

• Adds BuildJsonApiQueryAsync<T>() method that returns a JsonApiQueryResult<T> with the processed query, parsed parameters, and optional total count
• Introduces JsonApiQueryResult<T> class to encapsulate the query result
• Consolidates statistics/aggregations documentation from statistics-and-aggregations.md into a comprehensive build-query.md guide covering the new API and various use cases
• Adds 17 integration and unit tests covering filters, includes, sorting, count behavior, and edge cases

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
docs/docs/upgrade-guide.md	Documents v1.6.0 release with feature description, usage examples, and migration notes
docs/docs/toc.yml	Updates table of contents to replace statistics docs with new build-query guide and adds upgrade guide + contributing links
docs/docs/statistics-and-aggregations.md	Removes old statistics documentation (content migrated to build-query.md)
docs/docs/build-query.md	New comprehensive documentation covering BuildJsonApiQueryAsync usage, examples for exports/aggregations/projections, performance tips, and method comparison table
JsonApiToolkit/Models/Querying/JsonApiQueryResult.cs	New result class with Query, Parameters, and TotalCount properties using required init-only properties
JsonApiToolkit/Controllers/JsonApiController.cs	Implements BuildJsonApiQueryAsync method following similar patterns to JsonApiQueryAsync but without pagination
JsonApiToolkit.Tests/Controllers/BuildJsonApiQueryAsyncTests.cs	Comprehensive test suite with 17 tests covering basic queries, filters, includes, sorting, pagination behavior, and edge cases

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