fix: return single-chunk streamed content in Ollama create_stream

[HumphreySun98]

Why are these changes needed?

When a streamed Ollama completion produces exactly one content chunk and no
tool calls (a short response returned in a single chunk), the final-result
assembly in create_stream matches neither the tool-call branch
(len(content_chunks) > 0 and full_tool_calls) nor the text branch — which
required len(content_chunks) > 1 — and falls through to the else, setting
content to the empty tool-call list and completion_tokens to 0.

The text has already been streamed to the consumer chunk-by-chunk, but the
returned CreateResult reports content == [] and zero completion tokens, so
any consumer reading result.content from a short streamed response gets an
empty list instead of the text, and usage accounting under-reports.

This PR:

• changes the text branch to len(content_chunks) > 0, so single-chunk text
  responses return their content;
• reads completion_tokens from the final chunk's eval_count for all
  branches, which also fixes the token undercount for tool-call-only streamed
  responses (the old else hardcoded 0).

The existing multi-chunk streaming tests pass unchanged; a new regression test
covers the single-chunk case (which was previously untested — the existing test
always slices content into ≥2 chunks).

Related issue number

N/A — found during review of the streaming result assembly.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/ (none required for this change).
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed (ran ruff, mypy, and the relevant pytest locally).