feat: optional driver-owned table query templates

[debba]

Summary

• Add optional capabilities.table_query_templates and get_table_query_template for SELECT/UPDATE/DELETE previews.
• Delegate Generate SQL to the connection driver only when opted in; retain existing SQL generation for built-in drivers and older plugins.
• Keep CREATE TABLE inspection and all existing RPC signatures unchanged.
• Forward structured identifiers, schema, columns and the explicit SELECT limit to the plugin.
• Guard the modal against stale responses and show real errors instead of silently generating a different SQL dialect.

Backward compatibility

• Capability defaults to false and is omitted when serialized as false.
• The DatabaseDriver trait has a default implementation; existing drivers need no RPC implementation.
• Only a remote JSON-RPC -32601 selects the legacy fallback; transport errors, other error codes and malformed results propagate.
• Old hosts ignore the new plugin capability; no plugin minimum runtime increase is required solely for this optional feature.

Validation

• 105 frontend tests passed (SQL generation, optional templates, target modals and Command Palette).
• 136 Rust plugin-area tests and 2 driver-trait tests passed.
• TypeScript project build/typecheck and ESLint on changed source passed.
• GitNexus staged change analysis: 16 files, medium risk; changes limited to opt-in templates and command registration.

Coordination

Refs TabularisDB/tabularis-sqlserver-plugin#26.
Plugin pagination fix: TabularisDB/tabularis-sqlserver-plugin#30.
The matching plugin implementation is on feat/table-query-templates; this draft freezes the additive contract for joint review.

Tabularium

Before publishing the plugin, register the optional boolean table_query_templates (default false) under the driver-kind capabilities schema. Ingestion uses lenient AJV validation with removeAdditional: all, which otherwise strips the flag. No database migration, endpoint or SDK change is needed. Registry production configuration has not been modified.

[debba]

Matching SQL Server implementation: TabularisDB/tabularis-sqlserver-plugin#31 (stacked on the pagination fix in #30 of that repository). The new flag must be registered in the Tabularium driver-kind schema before publishing the plugin; registry code and production configuration remain untouched.