refactor(codersdk,cli,scaletest,site): add a shared workspace paging helper - #28028

Summary

Stacked on #28027.

Every caller that needed the full workspace list carried its own paging loop,
each with its own page size and termination rule. codersdk.AllWorkspaces now
owns that logic: it advances the offset by the requested page size and stops
once it reaches the total the response reports.

Callers routed through it: cli/list.go (QueryConvertWorkspaces),
cli/ssh.go (shell completion), cli/configssh.go, both cli/exp_scaletest.go
helpers, and all four sites in scaletest/prebuilds/run.go, including the two
poll loops that were not paginated at all. allWorkspacesForTemplate is
deleted.

The frontend gains the equivalent helper as API.getAllWorkspaces, exposed as
the allWorkspaces query and used by the agents pages. Its key carries an all
marker so it stays distinct from the single-page key while remaining matched by
workspace list invalidation.

support/support.go keeps its own loop: it needs the server-reported count for
the bundle and must stop early at --workspaces-total-cap.

Testing

• go test ./codersdk/ -run TestAllWorkspaces (new: single page, offset
  progression, short page continues, empty result, caller pagination overridden)
• go test ./cli/ -run 'TestList|TestConfigSSH'
• pnpm vitest run src/api/api.test.ts

Follow-ups

• support/support.go: request min(cap, pageSize) rather than a full page
  when --workspaces-total-cap is small; that also removes the
  all[:capTotal] trim.
• support/support.go: Run overwrites Count with the truncated length,
  contradicting the comment that says the server total is preserved.
• support/support.go: a 403 mid-loop yields a partial list alongside a
  full count, with only a warning log.
• The agents pages fetch every page on load and filter client side. A
  server-side organization: filter would avoid the fan-out.

This pull request was created by Coder Agents on behalf of @jscottmiller.