Expose and implement investigate_database includeCardinality

## Problem

`investigate_database` accepts `includeCardinality` in its Zod schema, but the MCP JSON schema does not advertise the property and disallows additional properties. The handler also never reads the value or returns cardinality data.

Schema-driven MCP clients therefore cannot request cardinality, while direct callers receive no cardinality data either.

## Expected behavior

Make `includeCardinality` an advertised, functional investigation option:

- include it in the MCP input JSON schema;
- conditionally collect/return cardinality data when requested;
- document any product-specific availability, limits, or warnings.

## Acceptance criteria

- Protocol/schema coverage confirms the property is advertised.
- Handler coverage proves false omits cardinality work and true returns the defined data/warnings.
- Product-specific unsupported cases are structured and non-fatal.

## Release assessment

Fixable in 1.4.0 if cardinality is still part of the advertised investigation contract. It is not critical; alternatively, remove the unused option from the Zod schema and documentation for this release.

Start at the investigate_database entry point, its Zod schema, and the MCP input JSON schema. Trace how includeCardinality is handled, then add protocol and handler coverage for false, true, and unsupported product cases. Done means the option is advertised, cardinality data and warnings are returned as defined, and unsupported cases are structured and non-fatal.

Written by the indexing model from the issue text.