The item view advertised its four selector alternatives independently of the
selected collection, while collection-specific constraints lived only in a
superRefine that does not survive JSON Schema conversion. Clients validating
against tools/list could not discover that modules accept only path/node_id
and sections/symbols accept only index/name.
Replace the refined object with a discriminated union on collection so the
converted draft-2020-12 schema rejects the same combinations the MCP server
rejects. Runtime selection semantics are unchanged.
Fixes#1695
Co-authored-by: Saim Shafique <sx4imm@gmail.com>
