Python: Fix Ollama model env var in documentation

[dmsuehir]

Motivation and Context

Running the python native Ollama examples fail when using the OLLAMA_CHAT_MODEL_ID environment variable mentioned in the documentation:

root ➜ .../samples/getting_started/agents/ollama $ export OLLAMA_HOST="http://localhost:11434"
root ➜ .../samples/getting_started/agents/ollama $ export OLLAMA_CHAT_MODEL_ID=qwen3:8b

root ➜ .../samples/getting_started/agents/ollama $ python3 ollama_agent_basic.py 
=== Basic Ollama Chat Client Agent Example ===
=== Non-streaming Response Example ===
Traceback (most recent call last):
  File "/workspace/agent_framework/agent-framework/python/samples/getting_started/agents/ollama/ollama_agent_basic.py", line 68, in <module>
    asyncio.run(main())
  File "/usr/lib/python3.12/asyncio/runners.py", line 194, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/asyncio/base_events.py", line 687, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/workspace/agent_framework/agent-framework/python/samples/getting_started/agents/ollama/ollama_agent_basic.py", line 63, in main
    await non_streaming_example()
  File "/workspace/agent_framework/agent-framework/python/samples/getting_started/agents/ollama/ollama_agent_basic.py", line 30, in non_streaming_example
    agent = OllamaChatClient().create_agent(
            ^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/dist-packages/agent_framework_ollama/_chat_client.py", line 102, in __init__
    raise ServiceInitializationError(
agent_framework.exceptions.ServiceInitializationError: Ollama chat model ID must be provided via model_id or OLLAMA_MODEL_ID environment variable.

When using OLLAMA_MODEL_ID instead of OLLAMA_CHAT_MODEL_ID, then the examples work:

root ➜ .../samples/getting_started/agents/ollama $ export OLLAMA_MODEL_ID=qwen3:8b
root ➜ .../samples/getting_started/agents/ollama $ python3 ollama_agent_basic.py 
=== Basic Ollama Chat Client Agent Example ===
=== Non-streaming Response Example ===
User: What time is it in Seattle? Use a tool call
Result: The current time in Seattle is 10:58 PM.

=== Streaming Response Example ===
User: What time is it in San Francisco? Use a tool call
Agent: The current time in San Francisco is 10:58 PM.

Description

This PR updates the environment variable name in the documentation and comments in the .py file. No actual code is being changed. I tested using OLLAMA_MODEL_ID with all 4 of the native ollama examples from main.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[dmsuehir]

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