feat: add bulk upsert by external_id (#102)

[dcfocus]

Summary

Adds a bulk / batch upsert (insert-or-replace by external_id) API across every layer, closing #102. Previously every upsert path was single-record, and add_many rejects existing external_ids, so there was no way to idempotently upsert a batch in one operation.

Stacked on #108 (issue #100). This branch builds on the indexed-validation work; review/merge #108 first. Until then this PR shows both commits — the batch-upsert commit is the last one. The diff will reduce to just #102 once #108 lands.

Core

ContextStore::upsert_many_by_external_id(records) -> Vec<UpsertResult> applies insert-or-replace per external_id in one logical operation, returning one result per input record (in order) indicating inserted vs. replaced, the resulting id, and the final version.

Semantics mirror the single-record upsert_by_external_id exactly:

• every record must carry a non-empty external_id;
• duplicate ids / external_ids within the batch are rejected (parity with add_many);
• validation is all-or-nothing — nothing is written if any record is invalid;
• caller-supplied supersession fields are ignored (replacement is managed by external_id);
• an insert whose external_id already exists on a non-tombstone but hidden row is rejected, exactly as a single insert is.

Composes with #100: a batch resolves existing external_ids in a single scan and validates id uniqueness via the indexed path (find_existing_keys), instead of full-scanning per record. The whole batch is written in one pass (single version bump where records share a shard). Supersession is resolved within the external_id-filtered set, which is correct for every flow that creates supersession through the public API (a successor keeps its predecessor's external_id).

Parity across surfaces

Layer	Addition
lance-context-api	ContextStoreApi::upsert_many + UpsertRecordsRequest / UpsertRecordsResponse / UpsertResultDto
lance-context-core	upsert_many_by_external_id + api_impl dispatch
lance-context (unified)	dispatch to local/remote
lance-context-client	RemoteContextStore::upsert_many + ContextClient::upsert_records
REST server	PUT /api/v1/contexts/{name}/records/batch
Python	Context.upsert_many(records, key="external_id") (+ pyo3 binding)

Tests

• core: insert, replace, mixed, idempotent re-application, within-batch duplicate id/external_id, missing/empty external_id, id collision with store, empty batch, and the BTree-indexed path.
• REST: insert+replace batch, empty rejected, missing external_id rejected.
• Python: python/tests/test_upsert_many.py (insert/replace/idempotent/mixed/within-batch dup/missing external_id/empty).
• README: bulk-upsert example added next to add_many.

Checks

• cargo test -p lance-context-core --lib — 65 passed (1 ignored bench)
• cargo test -p lance-context-server — 17 passed
• cargo fmt --all -- --check and cargo clippy --workspace --all-targets -- -D warnings — clean
• Python: new test_upsert_many.py (6) + test_add_many.py + test_external_id.py pass

Note: ~24 pre-existing Python test failures on main (mock _DummyInner.add() argument-count drift in the single add/upsert paths and S3 network tests) are unrelated to this change — this PR only adds upsert_many and does not touch those paths.

Acceptance criteria (#102)

• Upsert a batch keyed by external_id in one call (new inserted, existing replaced/superseded), single version bump where possible
• Per-record results indicate inserted vs. replaced and the resulting id
• Within-batch duplicate external_id handling defined and tested (rejected)
• Parity across Python / core / REST / client
• Tests cover insert, replace, mixed batches, and idempotent re-application
• Composes with Uniqueness validation full-scans the dataset on every write (O(n²) for incremental/bulk appends) #100 so batch upsert does not full-scan per record

Closes #102