fix(mem_wal): allow append-only tables without primary keys

[touch-of-grey]

Summary

• allow MemWAL initialization on append-only tables without unenforced primary key metadata
• keep bucket sharding constrained to the primary key when a primary key exists, but allow no-PK tables to bucket by a non-nested column
• update Rust, Python, and Java tests plus MemWAL docs

Fixes #6846

Tests

• cargo fmt --all
• cargo test -p lance test_initialize_mem_wal_bucket_sharding
• uv run make build
• uv run pytest python/tests/test_mem_wal.py::test_initialize_mem_wal_bucket_sharding_without_primary_key
• git diff --check

Java tests were not run locally because this machine has no Java runtime installed.

[claude]

Claude Code Review

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[codecov]

Codecov Report

❌ Patch coverage is 91.37931% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/mem_wal/api.rs	72.22%	4 Missing and 1 partial ⚠️

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[jackye1995]

thanks for the fix