feat: make embedding dimension configurable

[dcfocus]

Summary

• Add per-context embedding dimension configuration, defaulting to 1536 for compatibility.
• Infer the embedding width from existing dataset schemas on open, and reject mismatched explicit dimensions.
• Plumb embedding_dim through Python Context.create(...) and REST context creation.
• Add Rust and Python coverage for non-1536 stores and existing 1536 datasets.

Closes #73

Testing

• cargo fmt
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• uv run --project python --extra tests --reinstall-package lance-context pytest python/tests/test_persistence.py::test_custom_embedding_dimension_round_trips python/tests/test_persistence.py::test_retrieve_fuses_text_vector_and_filters -q
• uv run --project python --extra dev ruff format --check python/python/lance_context/api.py python/tests/test_persistence.py
• uv run --project python --extra dev ruff check python/python/lance_context/api.py python/tests/test_persistence.py
• cd python && /home/user/.local/bin/uv run --extra dev pyright
• git diff --check

Note: .codex/skills/ci-pr-helper/scripts/run_ci_checks.sh still points at rust/lance-context/Cargo.toml, which does not exist in the current repo layout, so I ran the equivalent current-layout checks directly.