Fix "fig.region" is not updated to the "region" of the current plot

[Chuan1937]

Track the current region at the PyGMT level, instead of relying on the GMT_Extract_Region function of GMT.

Fix method：

1.added the _current_region attribute to the Figure class to track the current region.
2.modified the region attribute to prioritize returning the tracked region, and if not set, revert to GMT extraction.
3.updated the current region in the basemap and coast methods.
4.added the _update_current_region method to handle region transformations.

Fixes #3764

[Copilot]

Pull request overview

This PR addresses issue #3764 by implementing region tracking at the PyGMT level instead of relying on GMT's GMT_Extract_Region function. The implementation adds a _current_region attribute to the Figure class and updates it when plotting functions are called with an explicit region parameter.

Changes:

• Added _current_region instance variable to track region at the PyGMT level
• Implemented _update_current_region() method in Figure class to update region tracking
• Added region tracking calls in basemap() and coast() functions

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
pygmt/figure.py	Added _current_region attribute, _update_current_region() method, and fallback logic in region property
pygmt/src/basemap.py	Added call to _update_current_region() to track regions set via basemap
pygmt/src/coast.py	Added call to _update_current_region() to track regions set via coast

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[seisman]

Not sure if this is the best way to address the issue. We need more brainstorming on potential solutions.