fix: Mark copy_with_new_sdk_key method as deprecated

[jsonbailey]

Note

Low Risk
Only adds a deprecation warning and documentation note for Config.copy_with_new_sdk_key, with no behavioral changes aside from emitting DeprecationWarning at call sites.

Overview
Marks Config.copy_with_new_sdk_key as deprecated by adding an explicit docstring notice and emitting a DeprecationWarning (via warnings.warn(..., stacklevel=2)) when the method is called.

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[jsonbailey]

Cursorbot called out this issue in a separate PR where it was not related to the work being done.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.