refactor(mcp): address Codex review — fix registry bug, DRY schema hints, remove column regex

P1.1 registry.py: move _plugins_loaded=True to after successful import so a
failed load doesn't permanently poison the registry.

P1.3 schemas.py: remove overly restrictive ColumnRef.name / FilterClause.column
/ BigNumberChartConfig.temporal_column regex that blocked valid column names
containing parentheses, slashes, and other SQL-common characters.

P2.3 (DRY): eliminate _CHART_TYPE_ERROR_HINTS second-registry in
schema_validator.py by adding schema_error_hint() to ChartTypePlugin protocol,
BaseChartPlugin default, and all 7 plugin classes. SchemaValidator now delegates
to the plugin registry instead of maintaining a parallel dict.

P3.3 test_registry.py: add full registry unit-test coverage (register, get,
all_types, is_registered, display_name_for_viz_type, proxy methods, duplicate
warning, empty chart_type validation, insertion-order guarantee).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>