fix: omit max_tokens from OpenAI requests when None (Azure compatibility)

[thuantan2060]

📝 Pull Request Summary

Fix Azure OpenAI compatibility by omitting max_tokens from API requests when the value is None. Azure OpenAI rejects null for this field, while standard OpenAI silently accepts it.

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✅ What does this PR do?

• Fixes 5 methods across 2 files that pass max_tokens=None directly to the OpenAI API, which serializes to
  max_tokens: null in the JSON payload.
• Changes the pattern from always including max_tokens in the request dict to only including it when the value
  is not None.
• Affected files and methods:
  • src/memu/llm/openai_sdk.py: chat(), summarize(), vision()
  • src/memu/llm/backends/openai.py: build_summary_payload(), build_vision_payload()
• No behavioral change when max_tokens is explicitly set — only the None/default case is fixed.

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🤔 Why is this change needed?

• Azure OpenAI's API returns a 400 Bad Request when max_tokens: null is included in chat completion
  requests:
  Invalid type for 'max_tokens': expected an unsupported value, but got null instead.
• Standard OpenAI's API silently ignores null and uses its default, masking the issue.
• This prevents memU from working with Azure OpenAI deployments (via the OPENAI_BASE_URL override), which is a
  common enterprise setup.
• Other backends (doubao, openrouter) and the http_client already handle this correctly with if max_tokens is not None guards — this PR aligns the OpenAI SDK client and OpenAI HTTP backend with that same
  pattern.

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🔍 Type of Change

Please check what applies:

• Bug fix
• New feature
• Documentation update
• Refactor / cleanup
• Other (please explain)

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✅ PR Quality Checklist

• PR title follows the conventional format (feat:, fix:, docs:)
• Changes are limited in scope and easy to review
• Documentation updated where applicable
• No breaking changes (or clearly documented)
• Related issues or discussions linked

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📌 Optional

• Edge cases considered
• When max_tokens is explicitly set to an integer: behavior unchanged (included in payload)
• When max_tokens is None (default): now omitted from payload instead of sending null
• Other backends (doubao, openrouter, http_client): already use this pattern, no changes needed
• Follow-up tasks mentioned
• The lazyllm_client.py backend may have the same issue but was not modified since it uses a different SDK
  interface — worth auditing separately