INTPYTHON-957: test upgrade path from legacy multikey index (#382)

[INTPYTHON-957](https://jira.mongodb.org/browse/INTPYTHON-957)

## Summary

Adds regression coverage for the index-upgrade path in
`MongoDBStore.__init__` introduced by [INTPYTHON-948 /
#375](https://github.com/langchain-ai/langchain-mongodb/pull/375). That
PR replaced a unique compound index on the `namespace` array (which
produced multikey collisions) with a unique compound index on a
denormalized `namespace_str` field, and added in-place migration logic
for collections created under the old schema. The migration logic itself
was untested — the existing `store` fixture wipes the collection and
drops all indexes before each test, so no test exercised the upgrade
branch. This PR closes that gap.

## Changes in this PR

Three new tests in
`libs/langgraph-store-mongodb/tests/unit_tests/test_store.py`, plus a
dedicated `legacy_collection` fixture (using its own
`long_term_memory_legacy` collection so it can't perturb or be perturbed
by the shared `store` fixture):

- **`test_upgrade_from_legacy_index`** — sets up a collection with the
legacy unique `(namespace, key)` multikey index and pre-existing
documents lacking `namespace_str`, then constructs `MongoDBStore` and
asserts: every document is backfilled with `namespace_str =
"/".join(namespace)`, the legacy index is dropped, a unique
`(namespace_str, key)` index now exists, reads return the original
values, and a `put` against a legacy `(namespace, key)` updates rather
than inserts.
- **`test_upgrade_with_conflicting_non_unique_index`** — creates a
non-unique `(namespace_str, key)` index up front, then asserts init
drops it and replaces it with the unique variant. Covers the explicit
branch at `base.py:218–224`.
- **`test_upgrade_idempotent`** — re-initializes `MongoDBStore` against
an already-migrated collection and asserts no document or index changes
between the two initializations.

No production code is changed.

## Test Plan

- All three new tests pass against a local Atlas instance: `uv run
pytest tests/unit_tests/test_store.py -k upgrade`.
- Full `test_store.py` (12 tests, 9 pre-existing + 3 new) passes: `uv
run pytest tests/unit_tests/test_store.py`.
- One side-effect surfaced during test development, flagged here for
visibility (not addressed in this PR): `base.py:230` drops the legacy
index by key spec, which pymongo resolves to the auto-generated name
`namespace_1_key_1`. Collections whose legacy index was created with a
custom name would not match, and the drop would raise `OperationFailure:
index not found`. Real-world risk is low because PR #375 created the
index without a custom name, but worth a separate ticket if we want
defense-in-depth.

## Checklist

### Checklist for Author

- [ ] Did you update the changelog (if necessary)? — N/A, tests only
- [x] Is the intention of the code captured in relevant tests? — these
tests *are* the intent
- [ ] If there are new TODOs, has a related JIRA ticket been created?
- [ ] Has a MongoDB Employee run [the patch build of this
PR](https://github.com/mongodb-labs/ai-ml-pipeline-testing?tab=readme-ov-file#running-a-patch-build-of-a-given-pr)?

### Checklist for Reviewer

- [ ] Does the title of the PR reference a JIRA Ticket?
- [ ] Do you fully understand the implementation? (Would you be
comfortable explaining how this code works to someone else?)
- [ ] Is all relevant documentation (README or docstring) updated?

Signed-off-by: Casey Clements <casey.clements@mongodb.com>