refactor: consolidate extra_body merge logic into ModelOption.merge_extra_body

[cptnm3]

Pull Request

Issue

Fixes #1615

Description

Promote _merge_extra_body to the public merge_extra_body static method and make it the single canonical implementation used by all three merge paths. Malformed chat_template_kwargs now raises TypeError instead of the old silent fallback.

Changes:

• model_options.py: rename _merge_extra_body → merge_extra_body; raise TypeError (naming the offending value and side) when chat_template_kwargs is not a dict; omit chat_template_kwargs from the output when the merged mapping is empty
• openai.py: rewrite _merge_user_extra_body as two chained merge_extra_body calls (default < base < user); update init call-site to public name
• litellm.py: replace inline hand-rolled merge block with a single merge_extra_body call; update seed call-site to public name
• test_model_options.py: replace silent-fallback tests with TypeError tests (base and overwrite side); add empty-both-sides-drops-key test; update no-mutation test to public name
• test_options.py: add cross-path delegation tests for Scenarios A–D across all four paths (_via_helper, _via_merge_model_options, _via_openai, _via_litellm); convert Scenarios A–C to async to include the LiteLLM path

Assisted-by: IBM Bob

Testing

• Tests added to the respective file if code was changed
• New code has 100% coverage if code was added
• Ensure existing tests and github automation passes (a maintainer will kick off the github automation when the rest of the PR is populated)

Attribution

• AI coding assistants used

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[planetf1]

LGTM. Consolidation does what it says — merge_extra_body is now the single implementation behind merge_model_options, LiteLLMBackend, and OpenAIBackend._merge_user_extra_body, and the new cross-path tests exercise all three call sites (not just the helper in isolation). Ran the test suite on this head: passing.

One non-blocking note: malformed chat_template_kwargs now raises TypeError instead of the old silent fallback — a real behaviour change on public ModelOption, worth a line in the PR description so it shows up in release notes, but no current caller depends on the old behaviour and the new failure mode matches the existing pattern in formatters/granite/base/util.py.

[cptnm3]

Hi @planetf1, Thanks for the review! I would like to bring to your attention that docs/versioned_docs/version-0.8.0/api/mellea/backends/model_options.mdx still references old private method _merge_extra_body. I have left it untouched hoping it will be reconstructed by the CI. I hope that's ok.
I'll add the line in PR noting the behaviour change.

[planetf1]

Hi @planetf1, Thanks for the review! I would like to bring to your attention that docs/versioned_docs/version-0.8.0/api/mellea/backends/model_options.mdx still references old private method _merge_extra_body. I have left it untouched hoping it will be reconstructed by the CI. I hope that's ok. I'll add the line in PR noting the behaviour change.

The version-0.8.0 docs won’t get updated by CI as they are intended specifically for the 0.8.0 version of the code. There we did have ModelOption._merge_extra_body

The 0.9.0 docs will get created by the release process, and they should pick up changes you make - the api docs are constantly rebuild during dev too so https://docs.mellea.ai/next will give you a build of docs from main