fix(prompts): use each non-dict placeholder item as message content in compile

[moneebullah25]

What does this PR do?

When a chat prompt placeholder is filled with a list containing non-dict items (for example ["a", "b"]), ChatPromptClient.compile appended one message per item but set every message's content to str(placeholder_value), the stringified whole list. It now uses str(msg), the current item, so each item becomes its own message.

Fixes #1971

Type of change

• Bug fix
• New feature
• Breaking change
• Refactor
• Documentation update
• Tooling, CI, or repo maintenance

Verification

List the main commands you ran:

uv run --frozen pytest tests/unit/test_prompt.py tests/unit/test_prompt_compilation.py -q
uv run --frozen ruff check langfuse/model.py tests/unit
uv run --frozen ruff format --check langfuse/model.py tests/unit/test_prompt.py tests/unit/test_prompt_compilation.py
uv run --frozen mypy langfuse

Results: 41 passed. 18 errors in tests/unit/test_prompt.py, all AttributeError: Langfuse client is not initialized raised from fixture setup; they are in tests this change does not touch (the changed parametrized case and the new test pass). The same 18 errors occur on main (40 passed, 18 errors there, without the new test), so they are pre-existing. ruff check, ruff format and uv run --frozen mypy langfuse are clean. The e2e tests were not run.

Checklist

• I self-reviewed the diff using code_review.md.
• I added or updated tests for behavior changes.
• I updated docs, examples, or .env.template if needed.
• I did not hand-edit generated files; if generated files changed, I used the upstream regeneration path.
• I did not commit secrets or credentials.

Confidence Score: 5/5

This PR appears safe to merge.

Summary

Fixes ChatPromptClient.compile so each non-dictionary placeholder item becomes its own message content, rather than repeating the whole list.

• Updates the mixed-list expectation.
• Adds a regression test for two string items.
• No actionable issues found. Tests were not run during this review.

_{Reviews (1) · Last reviewed commit: "fix(prompts): use each non-dict placehol..." · Reviewed by Greptile}

[claude]

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