stdlib: don't clear LogRecord.args before ProcessorFormatter processors run

[SarthakB11]

Summary

In the foreign branch of ProcessorFormatter.format(), record.args = () was being assigned before foreign_pre_chain and the main processors chain ran.
Processors that look at event_dict["_record"].args therefore always saw an empty tuple, even when stdlib was called with positional args like logger.info("a=%s b=%s", "a", "b").

The clear is necessary: once record.msg is replaced with the rendered structlog string, leaving non-empty record.args makes logging.Formatter.format() raise TypeError: not all arguments converted during string formatting.
But it only needs to happen after the processor chain.
The fix moves record.args = () to right before record.msg = ed / super().format(record) and leaves a comment so a future refactor doesn't drop it again.

test_clears_args (the regression guard for the original TypeError) and test_pass_foreign_args_true_sets_positional_args_key still pass.
New test_record_args_accessible_in_processor reproduces #771 at the public API surface and asserts the foreign-pre-chain processor sees record.args == ("a", "b").

Closes #771.

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