feat: bump Python minimum from 3.12 to 3.13, drop strenum backport (#14767)

Closes #14753

## What changed

| File | Change |
|---|---|
| `pyproject.toml` | `requires-python` → `>=3.13,<3.15`; remove
`strenum==0.4.15` |
| `Dockerfile` | `uv python install 3.13`, `uv sync --python 3.13` |
| `.github/workflows/tests.yml` | `uv sync --python 3.13` on both matrix
legs |
| `CLAUDE.md` | dev setup command + requirements note updated |
| `deepdoc/parser/mineru_parser.py` | `from strenum import StrEnum` →
`from enum import StrEnum` |
| `agent/tools/code_exec.py` | same |

`StrEnum` has been in the stdlib since Python 3.11 — the `strenum`
backport package is no longer needed once the floor is 3.13.

## Why uv.lock is not regenerated

`uv lock --python 3.13` fails because:

1. The infiniflow/graspologic fork pins `numpy>=1.26.4,<2.0.0`
2. `tensorflow-cpu>=2.20.0` (the first release with cp313 wheels)
depends on `ml-dtypes>=0.5.1`, which requires `numpy>=2.1.0`
3. These two constraints are irreconcilable on Python 3.13

The lockfile regeneration requires loosening the `numpy` upper bound in
the `infiniflow/graspologic` fork. Once that fork commit is updated and
the SHA in `pyproject.toml:49` is bumped, `uv lock --python 3.13` will
succeed.

## RFC corrections

Two claims in the original RFC (#14753) did not hold up under code
review:

- **"graspologic hard-blocks 3.13"** — the infiniflow fork at the pinned
commit has no `<3.13` Python constraint. The blocker is the transitive
`numpy<2.0.0` conflict with tensorflow-cpu's test dependency, not a
direct Python version cap.
- **"free-threading throughput gains for I/O-bound workload"** — Python
3.13 free-threading requires a special `--disable-gil` build and
provides no benefit for async I/O code (the GIL is already released
during I/O). The real motivation is forward compatibility and improved
error messages.

.github/workflows/tests.yml

@@ -158,7 +158,7 @@ jobs:
 
       - name: Run unit test
         run: |
-          uv sync --python 3.12 --group test --frozen
+          uv sync --python 3.13 --group test --frozen
           source .venv/bin/activate
           which pytest || echo "pytest not in PATH"
           echo "Start to run unit test"
@@ -222,7 +222,7 @@ jobs:
                # Patch entrypoint.sh for coverage
                sed -i '/"\$PY" api\/ragflow_server.py \${INIT_SUPERUSER_ARGS} &/c\   echo "Ensuring coverage is installed..."\n      "$PY" -m pip install coverage -i https://mirrors.aliyun.com/pypi/simple\n     export COVERAGE_FILE=/ragflow/logs/.coverage\n        echo "Starting ragflow_server with coverage..."\n        "$PY" -m coverage run --source=./api/apps --omit="*/tests/*,*/migrations/*" -a api/ragflow_server.py ${INIT_SUPERUSER_ARGS} &' ./entrypoint.sh
                cd ..
-               uv sync --python 3.12 --group test --frozen && uv pip install -e sdk/python
+               uv sync --python 3.13 --group test --frozen && uv pip install -e sdk/python
 
 
       - name: Start ragflow:nightly for Infinity