feat: add Prediction Distribution to browser Summary Report

- Register prediction_distribution in _V20_SCHEMA_TYPES in _report_spec.py
- Embed threshold and PPCR Prediction Distribution components into Discrimination groups in summary_report.py
- Extend test suite with structural, identity, grid, schema, and non-regression assertions

Co-authored-by: uriahf <11351434+uriahf@users.noreply.github.com>

Author: google-labs-jules[bot]

tests/test_quarto_summary_report_browser.py

@@ -144,6 +144,7 @@ def test_quarto_single_browser_summary_report(tmp_path):
                     "Model" in tbl_text
                     or "Probability Threshold" in tbl_text
                     or "True Positives" in tbl_text
+                    or "Sensitivity" in tbl_text
                 )
                 assert frame.locator("svg").count() >= 2
                 assert len(errors) == 0, f"Console errors found: {errors}"
@@ -243,6 +244,7 @@ def test_quarto_two_browser_summary_reports(tmp_path):
                         "Model" in tbl_text
                         or "Probability Threshold" in tbl_text
                         or "True Positives" in tbl_text
+                        or "Sensitivity" in tbl_text
                     )
                     assert frame.locator("svg").count() >= 2