feat(auth): Clear active organization context

[evanpurkhiser]

Adds a private POST /api/0/auth/config/clear-organization/ action that clears the session's active organization.

This lets Auth V2 remove the server-side post-login fallback when a user clears the organization context from the login page.

[github-actions]

Backend Test Failures

Failures on 19f7f10 in this run:

tests/sentry/management/commands/test_generate_controlsilo_urls.py::TestGenerateControlsiloUrls::test_no_missing_urls — log

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/management/commands/test_generate_controlsilo_urls.py:74: in test_no_missing_urls
    assert line in current_state, msg
E   AssertionError: 
E                 New control silo URL patterns detected!
E     
E                 The pattern:   new RegExp('^api/0/auth/config/clear-organization/$'),
E     
E                 Does not exist in the current pattern inventory. You should regenerate
E                 the pattern inventory with:
E     
E                 cd ../getsentry
E                 getsentry django generate_controlsilo_urls --format=js --output=../sentry/static/app/data/controlsiloUrlPatterns.ts
E     
E                 This command needs to be run in a getsentry environment
E                 in order to not lose patterns that are important for sentry.io
E                 
E   assert "  new RegExp('^api/0/auth/config/clear-organization/$')," in "// This is generated code.\n// To update it run `getsentry django generate_controlsilo_urls --format=js --output=/pat...gExp('^extensions/discord/link-identity/[^/]+/$'),\n  new RegExp('^extensions/discord/unlink-identity/[^/]+/$'),\n];\n"