Prep langchain-mongodb 0.12.0 release (#435)

Release prep for `langchain-mongodb` 0.12.0, per
[RELEASE.md](https://github.com/langchain-ai/langchain-mongodb/blob/main/RELEASE.md).
**Not the release itself** — this only bumps the version and dates the
changelog so the release workflow can be dispatched afterwards.

## Summary

- `libs/langchain-mongodb/pyproject.toml`: `version = "0.11.0"` →
`"0.12.0"`
- `libs/langchain-mongodb/CHANGELOG.md`: dates the existing 0.12.0
section `(XXXX/XX/XX)` → `(2026/08/24)`

Minor bump is appropriate — 0.12.0 carries new features, not just fixes:

- `MongoDBStructuredQueryTranslator` ISO 8601 date/datetime conversion
fix
- Auto-embeddings support in `MongoDBAtlasHybridSearchRetriever`,
`MongoDBAtlasParentDocumentRetriever`, `MongoDBAtlasSelfQueryRetriever`
- Native `$rerank` support across `MongoDBAtlasVectorSearch` and all
retrievers

## Changes in this PR

Two single-line changes, no source changes. Full diff is 2 files, 2
insertions, 2 deletions.

## Test Plan

`main` was verified this morning at `006027e` (the commit this branches
from) across the full CI matrix, simulating the minimum-version step
from #433:

| Lib | py3.11 | py3.13 |
|---|---|---|
| `langchain-mongodb` | 75 passed | 75 passed |
| `langchain-mongodb-deepagents-vfs` | 134 passed | 134 passed |
| `langgraph-checkpoint-mongodb` | 17 passed | 17 passed |
| `langgraph-store-mongodb` | 23 passed | 23 passed |

No behavioural change in this PR itself; CI on the branch is the gate.

## Two questions for the reviewer

**1. The changelog does not mention the dependency floor changes from
#431 and #432.** These are user-visible and can break installs for
anyone pinned below the new floors:

| Dependency | Change |
|---|---|
| `langchain-core` | `>=1.2.5` → `>=1.4.2` (runtime) |
| `pydantic` | newly declared, `>=2.13.0` |
| `numpy` | `>=1.26`, plus `>=2.3.2` on Python 3.14 via environment
marker |
| `typing-extensions` | `>=4.12.2` → `>=4.13.0` (dev only) |

Worth a changelog line before release, since a raised runtime floor is
the kind of change a consumer needs to see. Suggested wording:

> - Raise minimum dependency versions for Python 3.14 support:
`langchain-core>=1.4.2`, `pydantic>=2.13.0`, and `numpy>=2.3.2` on
Python 3.14 (unchanged at `>=1.26` on 3.11–3.13).

**2. The changelog date is `2026/08/24` (tomorrow).** `RELEASE.md` says
"including today's date". Intentional if the release is planned for
tomorrow — just confirming it isn't a slip.

## 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?