fix: make OLLAMA_BASE_URL optional with sensible default

[octo-patch]

Fixes #1087

Problem

base.py always reads os.environ["OLLAMA_BASE_URL"] with a hard dict
lookup, which raises KeyError for any user who runs Ollama on the
default localhost:11434 and hasn't set the environment variable.

A second related failure occurs when a caller already supplies base_url
in kwargs: the code also injects it from the env variable, causing
ChatOllama() got multiple values for keyword argument 'base_url'.

Both of these were reported in #1087.

Solution

• Replace os.environ["OLLAMA_BASE_URL"] with
  os.environ.get("OLLAMA_BASE_URL", "http://localhost:11434") so users
  running a stock Ollama install get the correct default without setting
  any environment variable.
• Only inject the base_url kwarg when the caller hasn't already
  supplied it, eliminating the "multiple values" error.

if "base_url" not in kwargs:
    kwargs["base_url"] = os.environ.get("OLLAMA_BASE_URL", "http://localhost:11434")

llm = ChatOllama(**kwargs)

Testing

1. Without OLLAMA_BASE_URL: get_llm("ollama", model="llama3") no longer raises KeyError.
2. With OLLAMA_BASE_URL: the env value is used as before.
3. With explicit base_url in kwargs: no "multiple values" error.

[assafelovic]

hey @octo-patch not sure why we need all that extra commits around images?