Filter advisor/mentor autocomplete dropdowns by role (#1126)

[jonfroehlich]

Closes #1126.

Problem

When an admin edits a Person and sets an advisor / co-advisor / mentor on an inline Position, the dropdowns offered every lab member regardless of role — e.g. undergrads as advisors (the original screenshots), a PhD student as co-advisor, an undergrad as mentor. There was save-time validation, but only after the editor had already picked an invalid option ("it shouldn't let me get that far").

Root cause

The plain advisor <select> was already role-filtered via PositionInline.formfield_for_foreignkey. But co_advisor and grad_mentor are autocomplete_fields: their options come from the admin autocomplete JSON endpoint (AutocompleteJsonView → the target model admin's get_search_results), which bypasses formfield_for_foreignkey entirely. So those two search boxes were never filtered.

Fix

Add PersonAdmin.get_search_results that reads the requesting Position field (model_name/field_name passed by the autocomplete view) and narrows results to the same role-appropriate querysets used for the plain dropdowns:

• advisor / co_advisor → get_active_professors_queryset()
• grad_mentor ("Mentor") → get_active_mentors_queryset() (senior members; professors are advisors, not mentors)

Tests

New integration tests in website/tests/test_advisor_mentor_autocomplete.py hit the real autocomplete endpoint and pin the role filtering for both fields. They fail before this change and pass after. All 33 related admin/person tests stay green.

Resolves all three parts of #1126

1. Advisor dropdown role-filtering — was already fixed (formfield_for_foreignkey).
2. co_advisor / mentor autocomplete role-filtering — fixed here.
3. Auto-focus the search box on open (follow-up comment) — now works out of the box via the Select2 bundled with the current Django; no change needed.

Screenshots

This is an admin-internal behavior change (no public-facing UI). Behavior is pinned by the new endpoint tests rather than screenshots — happy to add a before/after of the filtered autocomplete if preferred.

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