Fix notifier test payload API surface

[sorinc03]

Fixes #122.

Summary

• move the synthetic test_payload helper into the CLI, where test-webhook is the only caller
• remove test_payload from the txwatch-notifier public API
• add a focused CLI unit test for the generated test-webhook payload context
• drop the now-unused notifier chrono dependency and add the direct CLI dependencies needed by the helper

Validation

• git diff --check
• Not run: cargo fmt --check / cargo test because this environment does not have cargo, rustc, or rustfmt installed.