Document Fabric v3 QueryApproved query modes and add unit test (#208)

[Ishaan-Gpt]

Addresses #208.

This documents the Fabric v3 query capabilities on chaincode.Gateway.QueryApproved and adds a unit test where it adds value.

What changed:

• Godoc on QueryApproved now describes the Fabric v3 behavior: passing a zero sequence queries the latest approved chaincode definition (the peer resolves the latest sequence when the sequence field is omitted), and it notes that channel-wide queries go through the separate QueryApprovedChaincodeDefinitions lifecycle function.
• New unit test pins the zero-sequence pass-through: a 0 sequence reaches the peer unset in the marshaled QueryApprovedChaincodeDefinitionArgs, and the QueryApprovedChaincodeDefinition transaction function is invoked.

Verification notes:

• I confirmed the v3 semantics against the Fabric v3.0 lifecycle source: QueryApprovedChaincodeDefinition treats sequence 0 as latest, and the plural QueryApprovedChaincodeDefinitions function returns every approved definition on the channel.
• End-to-end coverage against a live Fabric v3 network still needs a running network, so that part is left as follow-up.

Tests run locally:

• go build ./... (clean)
• go test -race -count=1 ./pkg/... (all packages pass)
• go vet ./pkg/... (clean)
• gofmt -l -s (clean)