test: replace real-clock waits with mock clocks - #29458

Several tests waited on real wall-clock time. The httpapi heartbeat tests slept 3 x HeartbeatInterval (45s of slot time, 15s of package wall), TestEnterpriseCreateWithPreset waited ~31s per subtest for provisionerd's 30s backup poll (tests that call StoreReconciler.ReconcilePreset directly never publish the job-posted pubsub notification, because only Run drains that channel), and a handful of tests used time.Sleep.

This change drives the heartbeat tickers with a quartz mock clock (adding ServerSentEventSenderWithClock so the SSE sender's ticker is injectable; production callers are unchanged), posts the pending provisioner jobs on the reconciler's behalf in the preset test, and replaces the remaining sleeps with channel barriers or deterministic waits. TestConn_BlockEndpoints keeps its 10s wait: it is a negative check with no hook inside magicsock to know when disco has settled.

Before / After

Local, go test -count=1 -tags=testsmallbatch -parallel=8:

The remaining ~5s in coderd/httpapi is the coder/websocket close-handshake timeout against the raw net.Pipe mock client in two unrelated subtests; left for a follow-up with a real websocket client fixture.

Validation

• Targeted go test -count=1 -tags=testsmallbatch -parallel=8 on coderd/httpapi, enterprise/cli, coderd (TestWorkspaceActivityBump), coderd/x/chatd/mcpclient, scaletest/harness, tailnet: pass.
• -race -count=2 -shuffle=on on every converted test: pass. Repeated runs (-count=5 activity bump and mcpclient, -count=3 preset test): pass.
• golangci-lint run on all touched packages with a fresh cache: 0 issues.

Part of the test-suite speedup series (see #29457 for the package-ordering PR); independent of the other PRs.

Remote validation

Remote run on a dogfood workspace (separate compute), chat https://dogfood.cdr.dev/agents/5bbe73d0-866a-4841-9cc3-c5e5b4d97e44, tested head cc572708ed0 against base 9d973372774 on the same machine: coderd/httpapi 15.06s to 5.09s; TestEnterpriseCreateWithPreset subtests 31.7s to 1.06s each; TestWorkspaceActivityBump 4.00s to 0.95s; mcpclient 2.05s to 1.06s; scaletest/harness 3.54s to 0.56s. -race -count=3 -shuffle=on on all five packages: pass, no race reports. Endorsed verdict: PASS. Reviewer note from the run: this PR adds a small production seam (ServerSentEventSenderWithClock, 23 lines in coderd/httpapi/httpapi.go).

Xum (an AI agent) authored this PR on behalf of @ibetitsmike.