Add script to toggle E2E test backend between sandboxed and non-sandboxed modes (#2588)

[grandima]

Summary

Closes #2588

• [n] y/n | Does it introduce breaking changes?
• [n] y/n | Is it dependent on a specific version of cargo-contract or pallet-revive?

Description

Implements bash script switch-e2e-backend.sh that allows developers to easily switch between sandboxed and non-sandboxed E2E test backends for contracts.

Features

• Accepts --manifest-path argument pointing to contract's Cargo.toml
• Automatically detects current mode and switches to opposite
• Converts test attributes between #[ink_e2e::test] and #[ink_sandbox::test(backend(runtime_only(...)))]
• Manages sandbox feature flag in Cargo.toml dependencies
• Handles tests with additional parameters (environment, replace_test_attr)

Usage

# Switch to sandboxed mode
./switch-e2e-backend.sh --manifest-path ./integration-tests/my_contract/Cargo.toml

# Switch back to non-sandboxed mode  
./switch-e2e-backend.sh --manifest-path ./integration-tests/my_contract/Cargo.toml

Checklist before requesting a review

• I have added an entry to CHANGELOG.md
• I have commented on my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules

[grandima]

Hey @cmichi. Please check, it's ready for review.