Consolidate duplicate and nested imports in test files

[Copilot]

Several test modules imported the same modules multiple times, often inline within test function bodies rather than at the top of the file. This change hoists all imports to the module top and deduplicates them.

Changes

• tests/test_commands/test_builtin_commands.py: hoisted curses and ReclineCommandError; removed 8 inline import recline, 2 inline ReclineCommandError, and 2 inline import curses.
• tests/test_commands/test_async_command.py: hoisted os, sys, termios, threading, time, and the async_command symbols (CommandBackgrounded, CommandCancelled, set_terminal_echo); removed the corresponding inline duplicates.
• tests/test_repl/test_shell.py: hoisted readline, sys, typing.Annotated, ReclineCommandError, builtin_commands as bc, CommandCancelled, CLICommand, OutputFormatter, and _split_unquoted; removed ~17 inline duplicates (notably 8× import builtins and 3× _split_unquoted).

Net effect: ~93 lines removed, ~23 added; no behavioral changes.

Example

Before:

def test_fg_no_jobs(monkeypatch):
    import recline
    from recline.commands import ReclineCommandError
    monkeypatch.setattr(recline, "JOBS", {})
    ...

After (imports already at top of file):

def test_fg_no_jobs(monkeypatch):
    monkeypatch.setattr(recline, "JOBS", {})
    ...

Original prompt

In the test files, there are many instances of the same module being imported multiple times, and often not at the top of the module. Fix this

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