feat(tokens): achieve 80%+ unit test coverage for tokens service

[Rozerxshashank]

Fixes: #1487

I’ve spent some time digging into the token/services/tokens package and managed to push our unit test coverage from a pretty low 30% all the way up to 81.1%.

The main goal here wasn't just to bump up the numbers, but to make sure the core services are actually reliable. Here’s a breakdown of what I worked on:

• Better Mocks: I built out a much more robust mocking setup in mocks_test.go for the Vault, QueryEngine, TMS, and Network. I also added a couple of helpers (buildTestTMS, buildTestNetwork) that make it way easier to spin up isolated tests without a ton of boilerplate.
• Deeper Service Testing: I tackled the harder-to-reach logic in PruneInvalidUnspentTokens and CacheRequest. This includes covering those tricky parsing paths like Auditor flags and Graph Hiding that were previously untested.
• Database & Transactions: I expanded the tests for DBStorage to make sure we’re correctly handling the full transaction lifecycle: specifically things like Commits, Rollbacks, and how we notify other components.
• Bug Squashing: While writing these tests, I actually found and fixed a few potential nil-pointer issues in the service layer that could have popped up in edge cases.

Verification Results

• Test Status: All tests in token/services/tokens are passing (go test -v).
• Final Coverage: 81.1% of statements.

[Rozerxshashank]

Hi @adecaro , This PR is to the Fabric Token SDK! I’ve been working on increasing the unit test coverage for the token/services/tokens package, and I’ve successfully brought it from roughly 30% up to 81.1% .

[adecaro]

Hi @Rozerxshashank , great effort. I'll review ASAP. Thanks a lot 🙏

[adecaro]

Hi @Rozerxshashank , please, use counterfeiter for the mocks. We use it almost everywhere. Thanks 🙏