chore: consolidate Token mock into a single shared package

[salindne]

Background

The Token interface mock (mock_canton_token.go, 924 lines of mockery-generated code) is currently duplicated across multiple consumer packages:

• pkg/token/mocks/mock_canton_token.go
• pkg/transfer/mocks/mock_canton_token.go

PR #268 (AcceptWorker) adds a third copy at pkg/custodial/mocks/mock_canton_token.go. Once that PR merges, we'll have ~3000 lines of identical generated code across three locations, and any future consumer of Token will likely add a fourth.

Cost

• Every change to the Token interface (in pkg/cantonsdk/token/client.go) requires regenerating all copies — easy to forget one and end up with skewed mocks.
• ~3000 lines of identical generated code in the repo.
• Increases diff noise on PRs that touch the Token interface.

Proposed solution

Move the mock to a single shared location alongside the interface definition:

• pkg/cantonsdk/token/mocks/mock_canton_token.go

All consumer packages (pkg/token, pkg/transfer, pkg/custodial, and any future consumers) import from there.

Mockery's --srcpkg flag already supports this pattern. PR #268's pkg/custodial/accept_worker.go:21 go:generate directive uses it:

//go:generate mockery --srcpkg github.com/chainsafe/canton-middleware/pkg/cantonsdk/token --name Token --output ./mocks --filename mock_canton_token.go

The same directive can live once in pkg/cantonsdk/token/ and emit a single mock package.

Sequencing

This should land after PR #268 merges so we consolidate from the full known set of consumers. Otherwise we'd just have to redo the consolidation when #268 lands.

Out of scope

• Other interface mocks in the codebase — this issue is scoped to Token only. If similar duplication exists for other interfaces, those can be follow-ups.