feat: persist distance_metric in dataset schema metadata

[dcfocus]

Summary

Persists the configurable vector-search distance_metric (added in #74/#77) in
the dataset so it round-trips on open instead of being re-specified every
time. Closes #80.

Before this change distance_metric was a runtime-only option: a caller had to
pass the same metric on every open, and if they forgot, the store silently
fell back to the default l2 and ranked results differently from how the
dataset was intended to be queried — embedding_dim already recovers from the
schema, but the metric did not.

What changed

• Persist on create: the metric is written into the Lance schema
  metadata under lance-context:distance_metric (the same mechanism already
  used for lance-encoding:blob).
• Recover on open: distance_metric_from_schema reads it back and uses it
  as the store's metric, mirroring embedding_dim_from_schema.
• Mismatch guard: an explicitly passed metric that disagrees with the
  persisted one errors, reusing the existing embedding_dim mismatch-validation
  pattern in open_with_options.
• Backward compatible: datasets created before this change carry no key and
  default to l2.
• ContextStoreOptions.distance_metric is now Option<DistanceMetric> (None
  = use the persisted/default metric), matching embedding_dim: Option<i32>.
  Threaded through unified / server / PyO3 accordingly.

Tests

• Rust (store.rs): distance_metric_persists_across_reopen (create
  cosine, reopen without the option → cosine ranking), a mismatch-errors
  test, and a unit test that a metadata-less schema defaults to l2.
• Python (test_distance_metric.py): reopen-without-option keeps cosine
  ranking; reopening with a conflicting metric raises.

Verification

• cargo test -p lance-context-core — 41 passed
• cargo clippy --all-targets — clean; cargo fmt --check — clean
• pytest python/tests/ — 109 passed (the 2 S3 failures are pre-existing and
  environmental: no live bucket), ruff check / ruff format --check clean

Relationship

Follow-up to #74 / #77. Does not change the metric set or ranking math.

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