fix(backtest): ensure data_source is passed through all validation calls

- Pass data_source to derive_entry_spot in backtests.py
- Remove default 'synthetic' value for data_source in derive_entry_spot and validate_preview_inputs
- Update all tests to explicitly pass data_source parameter
- Improve error message with helpful suggestion for Databento/Yahoo Finance

app/pages/backtests.py

@@ -260,6 +260,7 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                 "GLD",
                 date.fromisoformat(DEFAULT_BACKTEST_START),
                 date.fromisoformat(DEFAULT_BACKTEST_END),
+                data_source="databento",  # Use databento for default dates
             )
         except ValueError:
             # Fixture source may not support the default date range
@@ -510,6 +511,7 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                     symbol,
                     parse_iso_date(start_input.value, "Start date"),
                     parse_iso_date(end_input.value, "End date"),
+                    data_source=str(data_source_select.value),
                 )
             except (ValueError, KeyError) as exc:
                 return None, str(exc)