fix(ollama): raise ollama floor to 0.6.3 for thinking levels (#40986)

Fixes #40985

---

`ChatOllama(reasoning="xhigh")` currently raises a Pydantic
`ValidationError` instead of reaching Ollama, because the `ollama` floor
still admits clients that reject model-defined thinking levels.

`ChatOllama.reasoning` is `bool | str | None` and `_chat_params()`
forwards the string verbatim as `think`. Since ollama 0.6.3
(ollama/ollama-python#744) `ChatRequest.think` is `Optional[Union[bool,
str]]`, matching the Ollama Go backend. But the declared range
`ollama>=0.6.1,<1.0.0` still admits 0.6.1 and 0.6.2, where the
annotation is `Optional[Union[bool, Literal["low", "medium", "high"]]]`
and `"xhigh"` or `"max"` fails validation client-side.

Verified both directions on the same snippet —
`ChatOllama(reasoning="xhigh")._chat_params(...)["think"] == "xhigh"`,
then constructing `ChatRequest(..., think=...)`:

- ollama 0.6.2 → `ValidationError` on `think.bool` and
`think.literal['low','medium','high']`
- ollama 0.6.3 → validates

This raises the floor only; no `ChatOllama` code path changes. Narrowing
`reasoning` inside `ChatOllama` instead would reject levels the Ollama
backend accepts and duplicate type logic the client now owns.

Per the PR template I'm opening this alongside the issue rather than
pre-approved, so please assign it to me if the approach looks right.

## Release note

`langchain-ollama` now requires `ollama>=0.6.3`, fixing a
`ValidationError` when `ChatOllama` is configured with a model-defined
reasoning level such as `"xhigh"` or `"max"`.

libs/partners/ollama/pyproject.toml

@@ -23,7 +23,7 @@ classifiers = [
 version = "1.1.0"
 requires-python = ">=3.10.0,<4.0.0"
 dependencies = [
-    "ollama>=0.6.1,<1.0.0",
+    "ollama>=0.6.3,<1.0.0",
     "langchain-core>=1.4.7,<2.0.0",
 ]