[CLN] Refactor grid selection logic by using active state checks

[Leguark]

Description

Refactored the regular_grid method to use the previously defined boolean variables dense_grid_exists_and_active and octree_grid_exists_and_active for consistency and improved readability, instead of directly checking if the grid objects are not None.

Relates to #grid-refactoring

Checklist

• My code uses type hinting for function and method arguments and return values.
• I have created tests which cover my code.
• The test code either 1. demonstrates at least one valuable use case (e.g. integration tests)
  or 2. verifies that outputs are as expected for given inputs (e.g. unit tests).
• New tests pass locally with my changes.

[Leguark]

• [CLN] Refactor grid selection logic by using active state checks #1062 👈 (View in Graphite)
• [ENH] Add name_id_map parameter to add_orientations and enable serialization validation #1060
• [CLN] Remove redundant octree level resolution check in grid initialization #1049
• [ENH] Add Greenstone example model and CG solver tests #1048
• main

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Graphite Automations

"Add gempy label" took an action on this PR • (09/25/25)

1 label was added to this PR based on Miguel de la Varga's automation.

[Leguark]

Merge activity

• Sep 30, 8:04 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Sep 30, 8:07 AM UTC: Graphite couldn't merge this pull request because a downstack PR [ENH] Add Greenstone example model and CG solver tests #1048 failed to merge.
• Sep 30, 8:10 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Sep 30, 8:14 AM UTC: Graphite couldn't merge this pull request because a downstack PR [ENH] Add Greenstone example model and CG solver tests #1048 failed to merge.
• Sep 30, 9:16 AM UTC: Graphite couldn't merge this PR because it failed for an unknown reason.