feat(catalogs): name the rerank operation and its billing basis

[jackspirou]

What

Adds ProviderOperationRerank, the tenth provider operation, so a catalog can state that a provider ranks documents by their relevance to a query.

Why the tag decides, and not the modalities

The package already holds a table of dedicated media operations keyed on input and output modalities. A reranker reads text and writes text, which is also the shape of a chat model, so the modalities separate nothing. The table therefore cannot carry the row.

ModelTagRerank does the work instead. A model carrying the tag serves the rerank operation and no chat operation. The same model without the tag stays a chat model. One test asserts both halves, because a reranker in the chat list would receive a request it cannot answer.

The two billing bases

Four providers publish a rerank endpoint and bill it in two different units. Cohere and OpenRouter bill a search unit. Jina and Voyage bill the tokens they read.

ModelOperationPricing gains SearchUnit and RerankBasis. The price validator holds the pair together:

Recorded basis	The validator requires
search-unit	a search_unit price
token	an input token price
absent	that no search_unit price is present
any other value	nothing, because it names the value and refuses

A price with no stated basis would leave a consumer to guess the unit, and a basis no constant names would reach a billing path as a silent default.

Scope

The endpoint contract lands here: an endpoint that names rerank validates, it still needs a path, and it accepts an author-specific path. No provider in the shipped catalog serves reranking today, so the endpoint data lands beside the providers that carry it in a follow-up.

Evidence

Command	Result
go test ./pkg/catalogs/...	ok
go test ./...	exit 0, 65 packages ok
go build ./...	clean
make docs-check	all documentation up to date
make openapi	regenerated, three enum entries added

Every command ran with GOTOOLCHAIN=go1.25.12, the toolchain continuous integration pins.

https://claude.ai/code/session_0131AtNgj1Qagv2csZGuxPFM