refactor(constants): derive multi-choice lists from Literal aliases

[LarytheLord]

Summary

Use typing.get_args() to derive runtime multi-choice constants from existing Literal aliases, so each choice set is declared once and reused for both typing and validation.

Closes #685

Changes

• added _literal_values() helper in pyinaturalist/constants.py
• moved multi-choice Literal aliases above runtime constants
• derived these constants from corresponding literal aliases: INBOXES, COMMUNITY_ID_STATUSES, CONSERVATION_STATUSES, GEOPRIVACY_LEVELS, HISTOGRAM_DATE_FIELDS, HISTOGRAM_INTERVALS, ID_CATEGORIES, ORDER_DIRECTIONS, PROJECT_TYPES, QUALITY_GRADES, SEARCH_PROPERTIES, SOURCES, and CC_LICENSES
• updated MULTIPLE_CHOICE_PARAMS['iconic_taxa'] to use a shared ICONIC_TAXON_CHOICES list
• removed the old TODO about Literal[*list] in this area
• added a regression test to ensure runtime choice lists stay in sync with their literal aliases

Validation

• uv tool run ruff check pyinaturalist/constants.py test/docs/test_signatures.py
• uv run pytest -q test/docs/test_signatures.py

[LarytheLord]

Pushed a small formatting-only follow-up in b6111c3 to fix the CI analyze job (ruff-format adjusted one blank line in constants.py).\n\nLocal validation after that commit:\n- uv tool run ruff check pyinaturalist/constants.py test/docs/test_signatures.py\n- uv run pytest -q test/docs/test_signatures.py (16 passed)