default maxTokens setting for autocomplete

[ferenci84]

Description

Default maxTokens at 256 for autocomplete if there is no overriding user setting for the model. Added autoCompleteMaxTokens

Change from this comment:
#3994 (comment)

Checklist

• The relevant docs, if any, have been updated or created - let's create a separate issue for this once the PR is approved and autoCompleteMaxTokens setting is kept in the completion options.
• The relevant tests, if any, have been updated or created - no relevant tests as far as I know

Testing instructions

I tested by directly putting a log message into the Ollama._streamFim() method.

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