chore: use explicit conversion flag in f-strings instead of str() (ruff RUF010)

[alexzhu0]

Summary

Ruff rule RUF010 flags f"{str(x)}" — the str() call is redundant because f-string interpolation already invokes __format__, which for the default format-spec defers to __str__. The explicit conversion flag !s makes the intent clear at the call site and drops one function call from the hot path of every format.

 except Exception as e:
     _log.error("搜索失败: %s", e)
-    return [types.TextContent(type="text", text=f"❌ 搜索失败: {str(e)}")]
+    return [types.TextContent(type="text", text=f"❌ 搜索失败: {e!s}")]

Semantic equivalence

str(x) and format(x, 's') both call x.__str__(). The !s conversion flag is specified by the format-string grammar (PEP 3101) to do exactly that:

The conversion fields cause a type coercion before formatting. Two conversion flags are currently supported: '!s' which calls str() on the value, and '!r' which calls repr().

For every exception object e, f"{str(e)}" and f"{e!s}" produce byte-for-byte identical output.

Scope

26 sites across 11 files. Almost all are error-handling paths — the common except Exception as e: ... f"failed: {str(e)}" pattern.

File	Count
m_flow-mcp/src/server.py	7
m_flow/api/v1/manual/manual.py	5
m_flow/memory/episodic/episode_size_check.py	3
m_flow/api/v1/maintenance/routers/get_maintenance_router.py	2
m_flow/cli/commands/memorize_command.py	2
m_flow/tests/unit/interfaces/graph/extract_graph_unit_test.py	2
m_flow/api/v1/memorize/routers/get_memorize_router.py	1
m_flow/retrieval/episodic/memory_fragment.py	1
m_flow/retrieval/utils/fine_grained_triplet_search.py	1
m_flow/retrieval/utils/procedural_memory_fragment.py	1
mflow_workers/workers/memory_node_saving_worker.py	1

11 files, +25 / −25.

Verification

$ ruff check . --exclude 'm_flow/baml_client' --exclude 'm_flow-frontend' --select RUF010
All checks passed!

$ for f in <each touched file>; do uv run python -c "import ast; ast.parse(open('$f').read())"; done
# clean across all 11 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