fix(coderd): reject redundant SSH autostarts of running workspaces - #29246

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An SSH client can read a workspace as stopped, then submit its autostart request after another client has already finished starting that workspace. The server rejects starts while a provisioning job is active, but accepts this delayed request once the previous start has succeeded. That creates an unnecessary new build and invalidates agent connections belonging to the previous build. Managed devcontainers can consequently lose connectivity or authorization even though the underlying workspace machine is still running.

Reject SSH autostart requests with HTTP 409 when the latest build is already a successful start. The existing SSH client's conflict handler refreshes the workspace and connects without requesting another start, preserving the running build and its agents. Normal autostarts of stopped workspaces and explicit starts or rebuilds remain available. This server-side change protects clients that already send the SSH connection build reason; it does not require a client upgrade or a per-workspace repair script, and it does not repair agents invalidated by earlier builds.

The regression test reproduces the delayed request deterministically and verifies that the latest build stays unchanged, while stopped-workspace autostarts and explicit starts still succeed. The running-workspace case fails before the fix on v2.37.0. The new regression, workspace builder tests, and existing SSH conflict-recovery tests pass with the fix on both v2.37.0 and upstream base 5e058aa.

--- FAIL: TestWorkspaceBuildSSHAutostart
    --- FAIL: TestWorkspaceBuildSSHAutostart/Running
        Error: An error is expected but got nil.
        expected: *codersdk.Error
FAIL github.com/coder/coder/v2/coderd 6.941s

$ go test -overlay=../postgres-test-overlay.json ./coderd ./coderd/wsbuilder -run '^(TestWorkspaceBuildSSHAutostart|TestBuilder_.*)$' -count=1
ok  github.com/coder/coder/v2/coderd     1.234s
ok  github.com/coder/coder/v2/coderd/wsbuilder 0.053s

ok  github.com/coder/coder/v2/coderd     6.631s
ok  github.com/coder/coder/v2/coderd/wsbuilder 0.068s

$ go test -overlay=../postgres-test-main-overlay.json ./cli -run '^TestSSH/(StartStoppedWorkspace|StartStoppedWorkspaceConflict)$' -count=1
ok  github.com/coder/coder/v2/cli 9.966s