feat(c): export vector-search projection over the C FFI

[JunRuiLee]

Purpose

The C FFI vector-search builder (bindings/c/src/vector_search.rs) exposed
with_vector_column / query_vector / limit / options / filter /
execute_read, but not projection. C callers therefore could not restrict the
materialized columns and always read every user column (plus the search score),
even though the Rust VectorSearchBuilder already supports with_projection.

Changes

• Add paimon_vector_search_builder_with_projection(builder, columns), mirroring
  paimon_read_builder_with_projection: columns is a null-terminated array of
  null-terminated C strings (pass null to clear). Names are stored on the
  builder state and applied in execute_read via with_projection.
• Column-name validation is deferred to execute_read (the core
  VectorSearchBuilder::with_projection is infallible and resolves the projection
  when the search runs), so an unknown projected column surfaces as an error from
  execute_read rather than from the setter.
• Add a compile-time C ABI signature guard for the new symbol.

Only execute_read is affected; the search-only paths are unchanged.

Tests

• vector_search_pk_projection_restricts_columns: projecting ["id"] through
  execute_read materializes only id + __paimon_search_score (the vector
  column is excluded), matching a Rust reference.
• vector_search_projection_unknown_column_errors_at_execute_read: an unknown
  projected column fails loud at execute_read.

cargo test -p paimon-c, cargo fmt --all --check, and
cargo clippy -p paimon-c --all-targets -- -D warnings all pass.