fix(instagram): paginate following endpoint to handle limit > ~200 (#1831)

[LeoLin990405]

Closes #1831.

What

opencli instagram following <user> --limit N issued a single fetch with the user-supplied limit passed straight through as count to /api/v1/friendships/{userId}/following/. Instagram returns HTTP 400 for count values above ~200, so any account followed by more than ~200 users could not be fully listed.

Fix

Replace the single fetch with a pagination loop:

• Fixed page size count=50 (safely under the API ceiling)
• Follow the next_max_id cursor
• Dedupe by pk (defends against cursor overlap)
• ~400 ms pacing between pages
• Terminate on: limit reached, missing cursor, empty page, repeated cursor, or a page that yields zero new unique users (avoids any infinite-loop edge if the API echoes itself)

Verification

• npx vitest run clis/instagram/instagram.test.js — 9/9 pass
• npm run test:adapter — 380 files / 3723 tests pass, no regression
• npm run build-manifest — idempotent (no CLI-surface change)

Tests added (clis/instagram/instagram.test.js, new file)

1. Single page < 50 users (no pagination)
2. 3 pages via next_max_id (rank continuity across 50+50+20)
3. Dedupe by pk on cursor overlap
4. limit respected — stops early, no extra fetch
5. Empty following list returns []
6. Repeated next_max_id → cursor-loop guard breaks (3 calls instead of 4)
7. Zero-growth page (page 2 dedupes to empty) → break
8. limit=0 short-circuits before the following endpoint
9. HTTP error mid-pagination propagates as Failed to fetch following: HTTP 429

Behavior

--limit	Before	After
20	20 (1 fetch)	20 (1 fetch)
200	works (1 fetch)	works (4 fetches)
300	HTTP 400	295 unique (issue: account follows 296)

CLI surface unchanged — this is a behind-the-scenes fix, no flag added, no README/manifest delta.