fix(ci): add tabulate to core dependencies

Windows Python 3.10 CI runs failed 4 tests in TestReportGeneration
and test_report_contains_new_sections with:

  ImportError: Missing optional dependency 'tabulate'.

pandas ``.to_markdown()`` dispatches to ``tabulate`` which pandas
lists as optional.  Our ``sp.causal(...).report(fmt='markdown'|'html')``
path uses ``.to_markdown()`` internally, so the dependency has to be
declared core on the StatsPAI side — otherwise a user who does

    r = sp.causal(...).report('markdown')

hits an ImportError at runtime on systems that happen not to pull
tabulate in transitively (Windows being the canonical case).

Adds ``tabulate>=0.9.0`` to ``[project].dependencies`` in
``pyproject.toml``.  Published sdist and wheel already shipped, so
this ships in v1.0.2 via the next PyPI release.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: brycewang-stanford

pyproject.toml

@@ -49,6 +49,11 @@ dependencies = [
     "openpyxl>=3.0.0",
     "xlsxwriter>=3.0.0",
     "python-docx>=1.0.0",
+    # Required by ``sp.causal(...).report(fmt='markdown'|'html')`` because
+    # pandas ``.to_markdown()`` dispatches to ``tabulate``.  Ship it as a
+    # core dep so published artifacts work on Linux/macOS/Windows without
+    # users having to discover the optional-dep pitfall at runtime.
+    "tabulate>=0.9.0",
 ]
 
 [project.optional-dependencies]