fix: route AsyncLogger output to stderr by default (fixes #1968)

[cgseyhan]

Problem

AsyncLogger writes its Rich progress output to stdout via the default Console() constructor. When crawl4ai is used as an MCP server over stdio (e.g. with mcp-crawl4ai, Claude Desktop, or mcp-inspector), the log lines are injected into the JSON-RPC stream. Clients fail to parse the messages and report:

Failed to parse JSONRPC message from server

This was reported in #1968.

async_logger.py – before

self.console = Console() # writes to stdout by default

ich.Console() without arguments writes to stdout. Library log output should go to stderr — this is the POSIX convention and what Python's own logging.StreamHandler does.

async_logger.py – after

self.console = console if console is not None else Console(stderr=True)

Two changes:

1. Default to stderr : Console(stderr=True) writes to sys.stderr, keeping stdout pristine for protocols that use it as a transport.
2. Injectable console parameter : callers that want stdout back (or need a custom target) can pass their own Console instance without monkey-patching.

Testing

Added ests/test_async_logger_stderr.py with 7 tests:

Test	What it checks
est_default_console_writes_to_stderr_not_stdout	capsys.out is empty after logging
est_default_console_is_stderr	logger.console.file is sys.stderr
est_custom_console_is_respected	Injected StringIO console receives output
est_stdout_console_can_be_injected	Legacy stdout behaviour can be restored
est_no_output_when_verbose_false	erbose=False still suppresses everything
est_file_logging_still_works	File logging path unaffected
est_stdout_clean_across_all_log_levels	End-to-end MCP scenario: stdout has only JSON

All 7 pass ✅

Backwards Compatibility

• Users who read console.file directly and expect stdout will see stderr instead. This is intentional and correct.
• Users who need the old behaviour can pass console=Console() (or Console(file=sys.stdout)) to the constructor.
• No other public API changes.