feat: structured disconnect attribution for agent logs (#25191)

Implements
[PLAT-60](https://linear.app/codercom/issue/PLAT-60/enhance-disconnect-logs-with-structured-reason-attribution):
adds structured disconnect attribution to disconnect logs throughout the
agent and tailnet packages.

Every disconnect log site now carries structured slog fields. All
existing logs remain; existing messages are preserved with the fields
added alongside.

New fields on disconnect log lines:

- `connect_type` — which layer disconnected: `server_to_agent`,
`agent_to_client`, or `client_to_server`
- `disconnect_reason` — categorical reason: `graceful`, `network_error`,
`server_shutdown`, etc.
- `disconnect_expected` — whether the disconnect is normal operation
(`true`) or should be investigated (`false`)
- `disconnect_initiator` — who started it: `client`, `agent`, `server`,
or `network` (control-plane sites only)
- `disconnect_detail` — free-form supplemental info (where useful)

## What's covered

**Control plane (`server_to_agent`):** coordination RPC, DERP map
subscriber, agent runLoop, agent Close, `BasicCoordination.Close`,
`Controller.run`.

**Data plane (`agent_to_client`):** SSH sessions, reconnecting PTY,
JetBrains port-forwarding.

<details>
<summary>Control-plane sites</summary>

| Site | Reason | Initiator |
|---|---|---|
| `agent/agent.go` `runLoop` EOF | `network_error` | `network` |
| `agent/agent.go` `runCoordinator` deferred exit | `server_shutdown` /
`graceful` / `network_error` | `agent` / `server` / `network` |
| `agent/agent.go` `runDERPMapSubscriber` deferred exit | same (shared
`classifyCoordinatorRPCExit`) | same |
| `agent/agent.go` `Close` shutdown timeout | `server_shutdown` + detail
| `agent` |
| `agent/agent.go` `Close` clean coord disconnect | `server_shutdown` |
`agent` |
| `tailnet/controllers.go` `BasicCoordination.Close` | `graceful` or
`network_error` | `c.initiator` |
| `tailnet/controllers.go` `Controller.run` `net.ErrClosed` |
`network_error` | `network` |

</details>

<details>
<summary>Data-plane sites</summary>

| Site | Reason | Notes |
|---|---|---|
| `agent/agentssh/agentssh.go` SSH session closed | free-form
(`graceful`, `process exited with error status: N`, etc.) | Also sets
`closeCause("normal exit")` for clean exits so coderd's
`connection_log.DisconnectReason` is no longer empty |
| `agent/reconnectingpty/server.go` PTY closed | `server_shutdown`,
error string, or `graceful` | |
| `agent/agentssh/jetbrainstrack.go` channel closed | `normal close` or
error string | Previously passed empty reason |

</details>

<details>
<summary>Bug fix</summary>

The deferred `disconnected from coordination RPC` log no longer fires
when the initial `Coordinate()` RPC call fails before any connection is
established.

</details>

Refs PLAT-60.

---

_This PR was prepared by Coder Agents on behalf of @Emyrk._
**Manually QA'd a lot of common disconnects**

---------

Co-authored-by: Coder Agents <noreply@coder.com>

agent/agent_internal_test.go

@@ -1,17 +1,20 @@
 package agent
 
 import (
+	"context"
 	"path/filepath"
 	"runtime"
 	"testing"
 
 	"github.com/google/uuid"
 	"github.com/stretchr/testify/require"
+	"golang.org/x/xerrors"
 
 	"cdr.dev/slog/v3"
 	"cdr.dev/slog/v3/sloggers/slogtest"
 	"github.com/coder/coder/v2/agent/agentcontextconfig"
 	"github.com/coder/coder/v2/agent/proto"
+	"github.com/coder/coder/v2/codersdk"
 	agentsdk "github.com/coder/coder/v2/codersdk/agentsdk"
 	"github.com/coder/coder/v2/testutil"
 )
@@ -86,3 +89,56 @@ func TestContextConfigAPI_InitOnce(t *testing.T) {
 	require.NotEmpty(t, mcpFiles2)
 	require.Contains(t, mcpFiles2[0], dir2)
 }
+
+func TestClassifyCoordinatorRPCExit(t *testing.T) {
+	t.Parallel()
+
+	canceled, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	cases := []struct {
+		name      string
+		ctx       context.Context
+		retErr    error
+		reason    codersdk.DisconnectReason
+		initiator codersdk.DisconnectInitiator
+	}{
+		{
+			name:      "local shutdown, no error",
+			ctx:       canceled,
+			retErr:    nil,
+			reason:    codersdk.DisconnectReasonServerShutdown,
+			initiator: codersdk.DisconnectInitiatorAgent,
+		},
+		{
+			name:      "local shutdown, with cleanup error",
+			ctx:       canceled,
+			retErr:    xerrors.New("close timed out"),
+			reason:    codersdk.DisconnectReasonServerShutdown,
+			initiator: codersdk.DisconnectInitiatorAgent,
+		},
+		{
+			name:      "remote graceful, no error",
+			ctx:       context.Background(),
+			retErr:    nil,
+			reason:    codersdk.DisconnectReasonGraceful,
+			initiator: codersdk.DisconnectInitiatorServer,
+		},
+		{
+			name:      "stream broke unexpectedly",
+			ctx:       context.Background(),
+			retErr:    xerrors.New("read: connection reset"),
+			reason:    codersdk.DisconnectReasonNetworkError,
+			initiator: codersdk.DisconnectInitiatorNetwork,
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			reason, initiator := classifyCoordinatorRPCExit(tc.ctx, tc.retErr)
+			require.Equal(t, tc.reason, reason)
+			require.Equal(t, tc.initiator, initiator)
+		})
+	}
+}