fix(errors): store cli_path as attribute on CLINotFoundError

[SHAI-Akshay-Tripathi]

What

CLINotFoundError previously accepted cli_path as a constructor argument
but only used it to format the error message — the path itself was not
retained as an attribute. This PR stores it as self.cli_path.

Why

The other custom error classes in src/claude_agent_sdk/_errors.py all
expose their structured arguments as attributes:

• ProcessError → self.exit_code, self.stderr
• CLIJSONDecodeError → self.line, self.original_error
• MessageParseError → self.data

CLINotFoundError was the lone exception, which meant callers wanting to
react to the missing-path case (e.g. log it, attempt a fallback location,
emit a metric) had to string-parse the message.

Example of what becomes possible:

​python except CLINotFoundError as e: if e.cli_path: log.warning("Claude CLI missing at %s", e.cli_path) ​

The message-formatting behaviour is unchanged, so this is fully
backward-compatible.

How to test

​bash python -m pytest tests/test_errors.py -v ​

The existing test_cli_not_found_error is extended with one assertion,
and a new test_cli_not_found_error_with_path covers the path-provided
case, mirroring the style of test_process_error.