Add add_subjects_to_subjectgroup to AsyncERClient

[victorlujanearthranger]

Adds three new methods to AsyncERClient:

add_subjects_to_subjectgroup posts to /subjectgroup/{id}/subjects/ to add a list of subjects to a subject group.

delete_subject and delete_source send DELETE requests to /subject/{id}/ and /source/{id}/ respectively. Both are irreversible — deleted records lose all linked observation history in EarthRanger.

[Copilot]

Pull request overview

This PR adds a new add_subjects_to_subjectgroup method to the AsyncERClient class, enabling users to add subjects to a subject group via a POST request to the subjectgroup/{group_id}/subjects/ endpoint.

Changes:

• Added add_subjects_to_subjectgroup(group_id, subjects) async method to AsyncERClient in client.py
• Added comprehensive test coverage for the new method, including success and error-handling scenarios (404, 401, 403, 500)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
erclient/client.py	New add_subjects_to_subjectgroup async method that POSTs subjects to a subject group endpoint
tests/async_client/test_add_subjects_to_subjectgroup.py	New test file with 5 tests covering success and HTTP error responses

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[chrisdoehring]

@victorlujanearthranger I added a remove function and added to the sync version for parity.