refactor(client)!: clean up house on client code

This commit:
  - Reorders methods into logical groups: transaction, block, address,
    script hash, and mempool.
  - Improves documentation throughout: clearer summaries, consistent
    style, and proper intra-doc links.
  - Moves `Sleeper`, `DefaultSleeper`, and `is_status_retryable` to the
    top of the file for better readability.
  - Updates return types from `Tx` to `EsploraTx` following the rename.
  - Removes unused `log` imports.
  - Updates copyright notice to `2020-2026`.

Author: luisschwab