[dajiaohuang]

Fixes #333.

vec0Update_Insert wrote rowid/chunk/vector data before vec0_write_metadata_value checked metadata types. An invalid metadata value could therefore return an INSERT error after partial writes. Validate all supplied metadata values before conflict replacement or row writes, and reuse the same type check in the metadata writer.

The regression test reproduces the explicit-transaction sequence with synthetic vectors: a valid insert, an invalid INTEGER metadata value, a later valid insert, and commit. It asserts that only successful rows remain, the rejected vector is not searchable, and its rowid is available for a corrected retry.

Validation on task-local Windows tooling:

• Built the loadable extension from the checkout and its vendored SQLite amalgamation with task-local Zig 0.16.0 (21 compiler warnings, all unused parameters/variable).
• pytest tests/test-loadable.py tests/test-metadata.py tests/test-insert-delete.py: 83 passed, 4 skipped; 144 snapshots passed.
• git diff --check and staged diff check passed.