feat: add contract decoding submissions API

[bh2smith]

What & why

Adds SubmitContracts and ListContractSubmissions to DuneClient, wrapping the new POST /v1/contracts/decode and GET /v1/contracts/submissions endpoints (docs: https://docs.dune.com/api-reference/contracts/introduction). Lets users submit contracts for decoding in batches of up to 100 with per-item results and idempotency keys, and poll status with cursor paging.

Architecture

• models/contracts.go - request and response types. ABI is a json.RawMessage since the API accepts an array or a JSON string; optional request fields are pointers with omitempty; ListContractSubmissionsOptions.ToURLValues() encodes only the set filters.
• dune/contracts.go - the two methods, following the datasets/uploads pattern; interface entries and URL templates in dune/dune.go.
• Library only: the CLI in cmd/ is a single-purpose query runner without subcommands, and the other endpoint families (datasets, uploads, matviews) are library-only too, so I kept parity with them rather than redesigning the CLI.
• Tested: httptest unit tests for body shape (ABI as array or string, omitted optionals), per-item result parsing, API error mapping, query encoding; e2e covers listing only, since every submit call creates a real submission for the key owner. go vet and -race tests pass. make lint cannot build golangci-lint 1.57.2 under my local Go 1.27, so CI is the authority for lint.
• README gains a Contract Decoding APIs section.

Fixes DF-1107

[TheEdgeOfRage]

Approved. One documentation suggestion left inline.