Minor: Refactor: unify OutputFormat handling via OutputFormat.resolve()

[jamesblackburn]

Add OutputFormat.resolve() static method for centralised, case-insensitive conversion of str/enum/None to OutputFormat. Replace ad-hoc .lower() comparisons throughout the read path with direct enum equality checks.

• options.py: add resolve() method, use it in output_format_to_internal, arrow_output_string_format_to_internal, and RuntimeOptions
• _store.py: use resolve() in _get_read_options_and_output_format and _adapt_frame_data; tighten return type to OutputFormat

[claude]

ArcticDB Code Review Summary

PR #3027 · Refactor: unify OutputFormat handling via OutputFormat.resolve()
Reviewed at: dca9212

API & Compatibility

• ✅ No breaking changes to Arctic, Library, or NativeVersionStore
• ✅ No deprecation protocol needed (additive change)
• ✅ No on-disk format changes
• ✅ No protobuf schema changes
• ✅ No key type / ValueType enum changes
• ❌ Subtle public behaviour change: RuntimeOptions.output_format and RuntimeOptions.set_output_format() now always store a resolved OutputFormat enum, never a raw string. Code that previously passed a string and later read back opts.output_format expecting a str (e.g. via == with a string literal) will now get an OutputFormat enum. This should be called out in the PR description.

Memory & Safety

• ✅ Python-only change; no C++ memory management involved

Correctness

• ❌ output_format_to_internal has no exhaustive fallback (flagged inline at options.py:225): OutputFormat.resolve() can return None when called with None (the default). If None propagates into output_format_to_internal(), all three if/elif branches are skipped and the function silently returns None, which will cause a confusing failure downstream when set_output_format(None) is called. Add an else: raise ValueError(...) guard, or assert the result of resolve() is non-None before passing it to this function.
• ✅ Exception handling in resolve() is correct — (ValueError, AttributeError) is the right catch for OutputFormat(value.upper()) on non-string inputs

Code Quality

• ✅ Removed all ad-hoc .lower() comparisons consistently
• ✅ Test class structure resolved (standalone functions used)

Testing

• ✅ Behavioural changes have corresponding tests in test_output_format.py
• ❌ test_output_format_to_internal_pyarrow has no availability guard: if pyarrow is not installed in the test environment this test will raise ModuleNotFoundError rather than skip. Add pytest.importorskip("pyarrow") or a skipif marker.
• ❌ No test for output_format_to_internal(OutputFormat.POLARS): the POLARS branch is exercised indirectly but there is no direct unit test equivalent to test_output_format_to_internal_pyarrow for Polars.

Build & Dependencies

• ✅ Pure Python change; no CMake, vcpkg, or dependency changes

Security

• ✅ No credentials, secrets, or input-validation regressions

PR Title & Description

• ✅ Title is clear and concise
• ✅ Description explains what changed and why
• ❌ Description does not mention the RuntimeOptions attribute behaviour change (str→enum) noted above under API & Compatibility

Documentation

• ✅ OutputFormat.resolve() has a docstring
• ✅ No public methods in library.py / arctic.py were changed
• ✅ _store.py changes are internal