[Issue] Complete Agent Inspector funnel telemetry and failure diagnostics

**Describe the bug**
Current BI can observe the `azure.ai.inspector` command through `ext.run`, but that only means the command started. It does not prove that the Inspector server started, the browser UI loaded, the UI connected to a running agent, or the user completed an interaction.

As a result, we cannot accurately measure Inspector intent, browser-launch success, agent-connection success, or explain where and why users drop from the funnel. A missing downstream event must be treated as **not observed**, not automatically as a failure.

Desired funnel:

```text
route selected
-> agent port ready
-> Inspector launch requested
-> Inspector ext.run started
-> Inspector server ready
-> browser open accepted
-> UI ready
-> agent connected
-> first request sent
-> first response received
```

**To Reproduce**
Run `azd ai agent run` for a non-Activity agent and inspect its telemetry. Existing `ext.run` data cannot distinguish the milestones above or diagnose most launch and connection failures.

**Expected behavior**
Use the existing extension `ReportUsage` API; no new azd-core telemetry events or attributes are required.

```text
extension.event = local_client.route.selected
ext.route = inspector | playground | suppressed

extension.event = inspector.funnel.stage
ext.stage =
ext.outcome = succeeded | failed
ext.reason =
```

Metric definitions:

- Inspector intent proxy: route selected with `ext.route=inspector`.
- Successful browser launch: SPA reports `setViewReady`; `browser.OpenURL` alone is insufficient.
- Agent connected: a successful application-level discovery/protocol request.
- Inspector used: first request forwarded to the agent.
- Successful interaction: first response or SSE data received.

Acceptance criteria:

- Emit each successful milestone at most once per invocation.
- Report known failures with stable, low-cardinality reasons.
- Never emit URLs, ports, paths, prompts, responses, IDs, or raw errors.
- Reuse existing Inspector `ext.run` for command start and final status.
- Verify Agents-to-Inspector correlation through `operation_Id`.
- Update telemetry documentation, tests, and the privacy checklist.

**Environment**
- Components: `azure.ai.agents`, `azure.ai.inspector`, and azd extension telemetry
- Platforms: all supported platforms

**Additional context**
`azure.ai.agents` owns route selection, launch checks, agent readiness, and launch request signals. `azure.ai.inspector` owns server, browser, SPA, connection, request, and response signals. The Inspector extension must upgrade to an azd SDK version that supports `ReportUsage`.

Start with the existing ReportUsage API and ext.run telemetry in azure.ai.agents and azure.ai.inspector, then trace the route, launch, UI, connection, request, and response milestones. Define stable low-cardinality outcomes and verify operation_Id correlation; done means the acceptance criteria pass, including tests, telemetry documentation, and the privacy checklist.

Written by the indexing model from the issue text.