feat: Support direct conversion between JSONB and jaq_json::Val

[b41sh]

This PR adds direct conversion support between JSONB and jaq_json::Val, allowing jaq execution to use jaq_json::Val without going through serde_json::Value.

Main changes:

• Added TryFrom<RawJsonb> for jaq_json::Val.
• Added TryFrom<&jaq_json::Val> for OwnedJsonb.
• Improved JSONB object/array builder paths to reduce unnecessary intermediate OwnedJsonb construction.
• Optimized JSONB iterators with accurate size_hint / ExactSizeIterator.
• Removed extra temporary allocation in ObjectIterator.

[Copilot]

Pull request overview

This PR introduces first-class interop between this crate’s JSONB representation and jaq_json::Val, enabling jaq execution to operate on jaq_json::Val directly (without a serde_json::Value hop). It also includes targeted performance/ergonomics improvements in JSONB builders and iterators to better support the new conversion paths.

Changes:

• Added TryFrom<RawJsonb> → jaq_json::Val and TryFrom<&jaq_json::Val> → OwnedJsonb conversions (new src/jaq.rs, exported via src/lib.rs).
• Optimized container iteration by adding accurate size_hint() implementations and ExactSizeIterator for array/object iterators.
• Refactored object building to avoid redundant map lookups when inserting keys (and added a build_from_entries path).

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/it/functions.rs	Small iterator usage cleanup in scalar key/value extraction tests.
src/raw.rs	Minor simplification of RawJsonb::len().
src/lib.rs	Exposes the new jaq module publicly.
src/jaq.rs	Implements JSONB ↔ jaq_json::Val conversions and adds focused unit tests for numeric edge cases.
src/core/databend/ser.rs	Removes unnecessary into_iter() when zipping values during object serialization.
src/core/databend/iterator.rs	Adds new_with_len, exact size_hint(), and ExactSizeIterator for JSONB iterators; refactors object iteration to remove temporary allocations.
src/core/databend/builder.rs	Refactors object builder insertion and adds build_from_entries; object build path refactoring impacts allocations.
Cargo.toml	Adds the jaq-json dependency.

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