INTPYTHON-1074 Drop the langchain-mongodb dependency from deepagents-vfs (#438)

[INTPYTHON-1074](https://jira.mongodb.org/browse/INTPYTHON-1074)

## Summary

`langchain-mongodb` was used for exactly one thing in
`langchain-mongodb-deepagents-vfs`: a single import of
`text_search_stage` at `search.py:16`, called once in the no-vector
fallback branch of `_grep_hybrid`.

The same file already imported the sibling helper from the other
package:

```python
from langchain_mongodb.pipelines import text_search_stage   # ← removed
from pymongo_search_utils import vector_search_stage
```

So two closely related functions were pulled from two different
packages, and one of those packages existed in the dependency tree
solely to supply it.

## Why the swap is safe

The two `text_search_stage` implementations are separate code, not a
re-export — but diffing the source shows the only differences are typing
syntax:

```diff
-    search_field: Union[str, List[str]],
+    search_field: str | list[str],
-    limit: Optional[int] = None,
+    limit: int | None = None,
-) -> List[Dict[str, Any]]:
+) -> list[dict[str, Any]]:
```

Same parameters, same defaults, same emitted stages (`$search` →
optional `$match` → optional `$set` score → optional `$limit`). For
contrast, `vector_search_stage` genuinely *is* the same object in both
packages (`langchain_mongodb` re-exports it from
`pymongo_search_utils`), which is why only `text_search_stage` needed
attention.

**No floor change needed.** `pymongo-search-utils` is already declared
at `>=0.3.0`, and `text_search_stage` has been present since 0.1.0 —
verified by installing 0.1.0, 0.2.0 and 0.3.0 and checking for the
attribute.

## Impact

Nine packages leave a runtime install:

```
async-timeout, greenlet, importlib-metadata, langchain-classic,
langchain-mongodb, langchain-text-splitters, lark, numpy, sqlalchemy
```

`langchain-mongodb`'s full transitive closure is 50 packages, but most
arrive anyway through `deepagents` (which pulls `langchain`,
`langchain-core`, `langsmith` and their trees) — so **nine is the honest
net figure**, not fifty. `numpy` and `sqlalchemy` are the notable
weight.

One nuance: `numpy` still appears in `uv.lock`, required by
`langchain-aws`. That is only reachable through the `[bedrock]` extra —
a base install no longer pulls it, confirmed below.

## Secondary benefit: removes release-order coupling

While planning the first deepagents-vfs release we had to reason about
whether it should ship before or after `langchain-mongodb` 0.12.0,
because the `>=0.11.0` floor named a version predating the Python 3.14
dependency fixes (INTPYTHON-1054, INTPYTHON-1065). That question
disappears permanently, along with one more sibling floor to keep
current.

## Test Plan

- Unit: **134 passed**. Integration: **26 passed**.
- Minimum versions (`--resolution=lowest-direct`, Python 3.11, run with
`--no-sync`): **134 passed** — worth checking explicitly, since the
resolved graph changes materially.
- `mypy` clean (16 source files); `ruff` and `ruff format` clean.
- Runtime-only install (`uv sync --no-default-groups`) confirms
`langchain-mongodb`, `numpy`, `sqlalchemy` and `lark` are all
**absent**.
- No `langchain_mongodb` references remain anywhere in the package
source.

## Timing

Worth landing **before** the first release. The package is unpublished,
so removing a dependency now costs nothing; afterwards it is a visible
change to consumers' resolved environments.

## Checklist

### Checklist for Author

- [ ] Did you update the changelog (if necessary)?
- [ ] Is the intention of the code captured in relevant tests?
- [ ] 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>