fix: Embed JSON-aliased VARCHAR columns as nested JSON (#38)

[jrosskopf]

Summary

Closes #38.

When a query returns a column whose DuckDB logical type is JSON, flAPI emitted the raw text as a JSON-escaped string in the REST response, forcing consumers to JSON.parse the value a second time. The reporter (BigQuery tree with recursive children arrays typed as JSON) correctly identified the dispatch site: QueryResult::convertVectorEntryToJson switches on duckdb_get_type_id(type), which returns the physical type id. DuckDB's JSON is a logical-type alias over VARCHAR, so the cell fell into the DUCKDB_TYPE_VARCHAR branch and got wrapped as a crow::json::wvalue(str).

Fix

• Inspect the logical type's alias via duckdb_logical_type_get_alias before dispatching VARCHAR. When it equals the literal "JSON" that DuckDB itself sets on LogicalType::JSON(), route to a new convertVectorJsonToJson helper that parses the bytes with crow::json::load and embeds the parsed value via the wvalue(const rvalue&) constructor. Nested objects and arrays now travel through the response unchanged. Malformed JSON degrades to the raw string rather than nulling the row.
• Destroy the logical type allocated at the top of convertVectorEntryToJson on every exit path. The dead duckdb_destroy_logical_type below the switch was unreachable, leaking one logical-type allocation per emitted cell (~24 bytes each, visible under ASan: removed 13/15 leak allocations the suite reports).

Both changes are scoped to src/query_executor.cpp and src/include/query_executor.hpp. The Arrow output path has the same physical-id dispatch in arrow_serializer.hpp but Arrow conventionally represents JSON via metadata/extension types, so it is intentionally out of scope for this PR.

Tests

Built TDD-style — failing tests landed first, then the fix.

Unit tests (test/cpp/query_executor_test.cpp, new [json] tag):

• Nested JSON object is embedded as Object (reproduces the issue exactly).
• JSON array column is embedded as List.
• CAST(NULL AS JSON) stays Null.
• Plain VARCHAR continues to render as a JSON string (regression guard).

Integration test (test/integration/test_json_column.tavern.yaml + supporting sqls/json_demo.{yaml,sql}):

• GET /json-demo/ returns the exact "Expected" shape from issue JSON-aliased VARCHAR columns are returned as escaped strings instead of nested JSON in API responses #38.

End-to-end verification with a debug-instrumented server confirmed the alias path fires (type_id=17 alias=JSON is_json=1) and the response body now matches the issue's expected output:

{"data":[{"id":1,"payload":{"a":1,"b":[10,20],"c":{"nested":true}}}],"total_count":1,"next":""}

Test plan

• New unit cases pass (flapi_tests "QueryExecutor JSON column" — 23/23 assertions).
• Full [query_executor] suite green (1092/1092 assertions, 7 cases).
• Full ctest suite — 586/587 pass; the lone failure (CachingFileProvider thread safety_test) is pre-existing and reproduces on stash-popped pre-change code.
• End-to-end response of GET /json-demo/ matches the issue's expected JSON.
• CI matrix (Linux x86/ARM64, macOS ARM64, Windows x64, flapii) — will be exercised by this PR.

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