External Dashboards API: support per-tile onClick drill-down

[alex-fedotyev]

Context

PR #2201 wires containers / tabs through the v2 external API but does not include per-tile onClick (the field that turns a tile cell into a navigation target — a search query or another dashboard). The shape exists in OnClickSchema in @hyperdx/common-utils and the UI persists it; the external API surface is missing.

Scope

• Mirror OnClickSchema (the discriminated union of OnClickSearch and OnClickDashboard) onto the v2 external API tile config schema.
• OpenAPI JSDoc with constraint parity (target id required, whereTemplate/whereLanguage optional, etc.).
• Conversion utilities propagate the field on create / get / update.
• Round-trip integration tests cover both OnClickSearch and OnClickDashboard shapes plus the absent (no onClick) case.

Out of scope

• MCP hyperdx_save_dashboard parity (separate follow-up).

Acceptance

• API consumers can configure tile-level click-through navigation programmatically.
• The persisted Mongo doc shape matches what the UI's chart editor produces today.