Set up a new test infrastructure with bdk_testenv

[j-kon]

Summary

This PR sets up a new integration test infrastructure for bdk-cli using bdk_testenv.

It replaces the dead shell-based integration test setup with a working direct-handler-based harness and adds a first funded-wallet flow test to prove the new path end to end.

What changed

• add bdk_testenv as a dev-dependency
• add a library target so integration tests can import CLI types and handlers directly
• expose the handler/backend pieces needed by the integration test harness
• replace the dead shell-based integration test setup in tests/integration.rs
• add a funded-wallet integration test that:
  • derives a receive address
  • funds it on regtest
  • syncs the wallet
  • verifies that balance updates

Why

The previous integration test setup was commented out and relied on an outdated shell-based approach. This PR establishes a minimal working foundation for direct handler-based integration testing, which matches the direction discussed in #120.

This is PR 1 of the work around #120. The OP_RETURN follow-up has already been prepared locally on top of this branch and will be opened as a separate PR after this one lands, to keep review focused and avoid restacking the PR 1 changes.

Validation

• cargo fmt --all --check
• BITCOIND_EXE=/usr/local/bin/bitcoind cargo clippy --all-features --all-targets -- -D warnings
• cargo test --all-features --test integration
• cargo test --no-run
• cargo test --no-default-features --no-run

Refs #120.

cc @tvpeter @va-an