feat!: label the VS Code family and report built-in app usage per app - #29906

`coder port-forward` reported usage with the deprecated
`UpdateWorkspaceUsageContext`.
It now reports a `port_forwarding` session, like `coder ssh` and the VS
Code SSH path, but only while a forwarded TCP connection is open.

**Behavior change:** CLI output is unchanged.
Port-forward sessions previously only bumped the workspace's last-used
time.
With the hidden `workspace-usage` experiment enabled, the server now
also records `port_forwarding` session counts under a new
`port_forwarding` family, which the deployment banner totals under
**Other**.
Without the experiment, `postWorkspaceUsage` returns 204 before reading
the body, so nothing changes.

<details>
<summary>Decisions</summary>

- Added `codersdk.UsageAppNamePortForwarding` (`port_forwarding`) rather
than reusing `ssh`, so port-forwarding isn't counted as interactive SSH.
The name matches the arbitrary app names RFC's connection-method name.
- Registered a `port_forwarding` family in `codersdk/appname.go`.
Families resolve at read time, so no data migration is needed. Template
insights list apps individually; telemetry, metricscache, and agentstats
gauges are legacy fixed fields and stay as they are. The Prometheus
insights family is left to #29906.
- Only TCP connections count. UDP forwards never see EOF, so one
datagram would keep the session active until exit.
- Reports once at startup, like the old call, then every minute and once
on shutdown. An idle tick sends an empty body, which still bumps
last-used.

</details>

Closes [CLI-31](https://linear.app/codercom/issue/CLI-31)

_This PR was generated by Coder Agents on behalf of @nickvigilante._


<details>
<summary>CI note: unrelated test-go-race-pg flake</summary>

The `test-go-race-pg` job failed on a data race between
`tailnet.(*configMaps).updatePeerLocked` and slog formatting of the same
peer value, which does not touch port-forward code. The job was rerun.

</details>

Report template insights session usage per app instead of five fixed
family rows, with registry names and icons.

- Add per-app rows for reported session apps (VS Code, Cursor, GoLand,
  Zed, etc.); unused apps are omitted
- Replace `type` (`builtin`/`app`) with `method` (`ssh`,
  `reconnecting_pty`, `port_forwarding`, `workspace_app`); session
  methods are derived from the app name until #31012 stores them
- Add `usage_total_seconds` to count each user's active minutes once,
  and use it as the UI bar denominator so overlapping apps can exceed
  100%
- Merge template apps sharing a slug into one row across templates,
  using the newest display name and icon
- Rename Web Terminal slug from `reconnecting-pty` to
  `reconnecting_pty`
- Cap the response at the 64 busiest unregistered apps and fold the
  rest into "Other apps", flagged with `seconds_is_upper_bound`
  ("Up to" in the UI)
- Add `unknown` and `overflow` accounting rows with explanatory
  tooltips, and a fallback icon for apps without one
- Remove the unused `codersdk.UnionByFamily` helper from the Go SDK

BREAKING CHANGE: `report.apps_usage` in `GET /api/v2/insights/templates`
no longer returns the fixed five rows, family slugs, or previous display
names. Consumers must update.

Part of CLIENT-642
Prometheus metrics handled separately in #29906