fix: guard unsupported endpoint searches

[TDanks2000]

What

• narrow IGDBSearchableEndpointPath to endpoints actually marked searchable
• make the high-level IGDBEndpoint.search() helper reject unsupported endpoint paths
• add regression and type coverage
• correct the endpoint docs and invalid Companies search example
• add a patch changeset

Why

IGDB only documents search support for Characters, Collections, Games, Platforms, Themes, plus the dedicated Search endpoint. The wrapper exposed the helper uniformly, so calls such as client.companies.search(...) appeared valid but produced an unsupported IGDB request.

Official reference: https://api-docs.igdb.com/#search

Impact

Unsupported high-level searches now fail early with IGDBValidationError, while supported endpoint searches retain the same API.

Checks

• regression coverage for supported and unsupported search helpers
• @ts-expect-error coverage verifies the searchable path type is genuinely narrowed
• compared branch against main for scope
• local bun run check was not available in the connector-only environment; CI should be treated as the executable verification