[Docs] Add comprehensive guide for testing cross-contract calls with MockEnv

[ekelemepraise-code]

Summary

Cross-contract call testing is one of the most requested documentation topics but currently only has a bare-bones example in the examples/cross-contract/ directory. The README does not explain the setup pattern at all.

What's Needed

A full guide section covering:

1. Registering multiple contracts in a single MockEnv
2. Passing contract IDs between contracts in tests
3. Asserting events from inner contracts when an outer contract triggers them
4. Authorization chains — how mock_all_auths propagates through cross-contract calls
5. Debugging tips — reading nested call traces

Proposed Structure

Add a new section to README.md:

### Testing Cross-Contract Calls

#### Registering Dependent Contracts
...

#### Routing Contract IDs
...

#### Event Assertions in Nested Calls
...

Also expand examples/cross-contract/src/test.rs with a realistic multi-level call scenario (e.g., Aggregator → AMM Pool → Token).

Acceptance Criteria

• README section added with working code examples
• examples/cross-contract test expanded
• All code examples compile with cargo test