chore: drop redundant "r" mode argument to open() (ruff UP015)

[alexzhu0]

Summary

Ruff rule UP015 flags open(path, "r", ...) where the mode argument is redundant — Python's open() defaults to mode="r" (text-read) when the argument is omitted, so dropping the literal "r" is byte-identical. 9 occurrences, all auto-fixable.

-        with open(self.dataset_path, "r", encoding="utf-8") as f:
+        with open(self.dataset_path, encoding="utf-8") as f:
             for line_num, line in enumerate(f, 1):

Semantic equivalence

Python's open() docs state that mode defaults to "r":

The default mode is 'r' (open for reading text, a synonym of 'rt').

Removing the explicit "r" leaves text-read mode unchanged. No positional/keyword argument shift either — encoding is already keyword-only in every touched site.

Scope

9 files, 9 sites, +9 / −9 (pure argument removal).

File	Sites
examples/episodic_stage5_smoke_test.py	1
m_flow/debug/trace_cli.py	1
m_flow/eval/config.py	1
m_flow/eval/loader.py	1
m_flow/eval/report.py	1
m_flow/llm/prompts/read_query_prompt.py	1
m_flow/shared/loaders/core/csv_loader.py	1
m_flow/shared/loaders/core/text_loader.py	1
m_flow/tests/tasks/descriptive_metrics/metrics_test_utils.py	1

Verification

$ uv run ruff check . --exclude 'm_flow/baml_client' --exclude 'm_flow-frontend' --select UP015
All checks passed!

$ for f in $(git diff --name-only main); do uv run python -c "import ast; ast.parse(open('$f').read())"; done
# clean across all 9 files

No behavior change. Generated code under m_flow/baml_client/ is untouched.

I affirm that all code in every commit of this pull request conforms to the terms of the M-flow Developer Certificate of Origin