feat(ffi): expose clustering domain metadata read path

[chiinlquah]

Add get_clustering_domain_metadata, an FFI symbol that returns the JSON configuration of the table's delta.clustering domain (or NULL when absent). Mirrors get_domain_metadata but routes through Snapshot::get_domain_metadata_internal, bypassing the user-facing delta.*-prefix guard.

Motivation: writers produce a delta.clustering DomainMetadata when creating a clustered table, but the existing FFI get_domain_metadata rejects reads on delta.*-prefixed domains. There's no FFI-exposed way to recover what an FFI-exposed writer wrote. External engines composing their own resolved clustering descriptors end up with a one-way wall.

The internal Rust path (Snapshot::get_physical_clustering_columns) already does the right thing for in-process use; this just gives external engines the same access.

Tests:

• test_get_clustering_domain_metadata -- clustered table; verifies the new symbol succeeds while get_domain_metadata still rejects.
• test_get_clustering_domain_metadata_absent -- unclustered table; verifies NULL return.

cargo test -p delta_kernel_ffi --lib: 142/142 pass.
cargo clippy -p delta_kernel_ffi --all-targets -- -D warnings: clean.

What changes are proposed in this pull request?

How was this change tested?

[codecov]

Codecov Report

❌ Patch coverage is 99.41520% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.75%. Comparing base (1ca53fc) to head (b628509).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ffi/src/domain_metadata.rs	99.37%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2573      +/-   ##
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+ Coverage   88.66%   88.75%   +0.08%     
==========================================
  Files         181      182       +1     
  Lines       61488    62184     +696     
  Branches    61488    62184     +696     
==========================================
+ Hits        54520    55190     +670     
+ Misses       4874     4854      -20     
- Partials     2094     2140      +46     

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