fix: Serialize VARINT/BIGNUM, GEOMETRY and VARIANT columns (#89)

[jrosskopf]

Summary

Closes the last gap from the #89 type audit. VARINT/BIGNUM, GEOMETRY, and VARIANT previously fell through to the default case and serialized as null. They now produce real values.

Instead of hand-writing a decoder per type, this uses a universal C++-API fallback: a C-API duckdb_vector handle is just a reinterpret_cast of duckdb::Vector*, so we borrow the C++ Vector and let DuckDB's own Value stringification do the work.

Type	Before	After
VARINT / BIGNUM	null	exact decimal string (lossless arbitrary precision)
GEOMETRY	null	DuckDB text form, e.g. "POINT (1 2)" (spatial loaded)
VARIANT	null	nested JSON when its rendering is JSON, else its string form
any future/unknown type	null	best-effort string

The whole fallback is wrapped in try/catch, so an unconvertible extension value degrades to null rather than crashing.

Notes

• 128-bit / arbitrary-precision integers are strings (consistent with the HUGEINT decision in fix: Correct REST JSON serialization of UUID/HUGEINT/BLOB/BIT (#89) #92) so values beyond 2⁵³ survive JSON consumers.
• A struct-origin VARIANT stringifies as DuckDB's SQL-ish {'k': v} (not JSON), so it serializes as a string; JSON-derived VARIANTs embed as nested JSON. Documented in code + tests.

Test plan

• New [query_executor][exotic_types] tests: VARINT (negative/large/multi-row/NULL), VARIANT (object embed, scalar, struct-origin, NULL).
• GEOMETRY verified manually (POINT (1 2)); not in committed tests since it needs the spatial extension download.
• Full C++ unit suite: 645/645 passing.
• Reviewed with codex (LGTM — cast valid, no leaks, never emits invalid JSON).

Relates to #89