Return all workers from scheduler_info() by default

[mrocklin]

Client.scheduler_info() defaulted to n_workers=5, silently truncating the "workers" dict to the first five workers. This was misleading and, more importantly, broke restart_workers: both the nanny-required validation and the worker name -> address resolution read this truncated set, so on clusters with more than five workers the nanny check was silently skipped and name-based restarts of later workers misfired.

Changes:

• scheduler_info() now defaults to n_workers=-1 (all workers). This only affects explicit, on-demand calls, not the periodic per-client poll, so the scheduler-scalability fix from Reduce size of scheduler_info #9045 / Scheduler not robust to many connected clients #9043 is not reintroduced (the periodic cache populator stays capped at 5).
• _restart_workers now fetches a fresh, full identity via scheduler.identity(n_workers=-1) and performs both the nanny check and the name resolution there, correct for any cluster size. This also collapses the previous two identity RPCs on the sync path into one.

Adds a regression test that restarts a non-nanny worker beyond the historical five-worker cap; it raises the expected error only with the fix.

xref #9065

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    40 files  ± 0      40 suites  ±0   14h 24m 21s ⏱️ - 12m 53s
 4 154 tests + 1   3 973 ✅ ± 0    178 💤 ±0  3 ❌ +1 
80 840 runs  +19  76 600 ✅ +19  4 237 💤  - 1  3 ❌ +1 

For more details on these failures, see this check.

Results for commit 9caa747. ± Comparison against base commit 3b0ba32.