This reverts commit fe092f3.

Add opt-in pagination to the existing /repos/ endpoint, triggered
when a caller sends the per_page query param without a search query.
Existing consumers that don't send per_page continue to receive the
full repo list unchanged.

The pagination interface is provider-agnostic:
- BaseRepositoryIntegration.get_repositories_paginated() returns None
  by default (provider does not support pagination)
- GitHubIntegration overrides it to slice the Django-cached repo list
- The endpoint dispatches based on the return value, falling back to
  get_repositories() when pagination is not supported

This supports the SCM onboarding repo selector, which needs fast
page-at-a-time loading for orgs with many GitHub repos.

Refs VDY-46

Promote the local to_repo_info closure to a method on
GitHubIntegration so both get_repositories and
get_repositories_paginated share the same formatting logic.

Refs VDY-46

Clamp per_page to at least 1 to prevent infinite empty pages
(per_page=0) or incorrect has_next (per_page=-1). Add docstring
to get_repositories_paginated noting it always serves from the
cached accessible-repos list.

Refs VDY-46

Wrap get_repositories_paginated in try/except for IntegrationError
and IdentityNotValid so token revocations return 400 instead of 500.

Validate per_page as an integer (non-numeric input like ?per_page=abc
defaults to 100 instead of raising ValueError).

Clamp cursor offset to >= 0 so crafted cursors like 0:-5:0 cannot
produce unexpected slicing behavior.

Refs VDY-46

Add tests for input validation edge cases: per_page=0, per_page=-1,
per_page=abc, per_page=200, negative cursor offset, and
IntegrationError in the paginated path.

Document that accessibleOnly is ignored in the paginated path and
that CursorResult is only used for Link header generation. Remove
redundant return_value=[] from test patch decorators.

Refs VDY-46

Document cache consistency caveat in get_repositories_paginated
docstring: cache expiry between pages can cause duplicates/skips,
acceptable for infinite-scroll.

Make _parse_cursor a staticmethod. Add comment clarifying per_page
and offset are guaranteed set when paginated is not None.

Refs VDY-46

Fix references to the old get_accessible_repos_cached name in
get_repositories_paginated and pagination tests after rebase.

…icing

Paginated repo browsing now makes a single GitHub API call per page
via GET /installation/repositories?page=N&per_page=M instead of
fetching all repos into a cache and slicing. This avoids loading
the full repo list into memory on every request.

Also switches to the built-in get_per_page() and
get_cursor_from_request() helpers, raises NotImplementedError in the
base class instead of returning None, and removes the custom
_parse_cursor method.

Refs VDY-46

Document that archived repos and installableOnly filtering can cause
pages to have fewer items than per_page and has_next to overestimate.
Add test for NotImplementedError fallback when a provider does not
support paginated browsing.

Refs VDY-46