Feature Request: session-scoped tool inventory API (list MCP-discovered tools loaded into a session)

## Problem

No public API enumerates the tools loaded into a *session* — built-ins merged with MCP-discovered tools from `SessionConfig.mcpServers`. Closest existing surfaces don't cover it:

- `client.rpc.tools.list({ model? })` is server-scoped — built-ins only, no session-loaded MCP tools.
- `session.rpc.mcp.list()` returns server connection status (`name`, `status`, `error?`, `source?`), not tools or counts per server.
- `session.mcp_servers_loaded` event fires once on session startup but carries the same server-status-only payload — no tool list.
- `session.mcp_server_status_changed` carries `{ serverName, status }` only.
- `session.tools_updated` event payload is just `{ model: string }`.

A session can report `mcp.status: "connected"` and still expose zero tools (wrong CLI flags, version mismatch, schema error) — there's no SDK-side way to detect this.

## Why it's needed

- **Operational visibility.** Confirm before the first turn that an MCP server actually contributed the expected tools, not just that its process started.
- **Diagnostic UIs.** Surface per-server tool counts to operators of BYOK / multi-MCP deployments.
- **Test assertions.** Verify a given MCP wiring exposes the expected tool set without spawning a parallel `tools/list` probe.
- **Parity with prior MCP clients.** Other MCP integrations (e.g. `@ai-sdk/mcp`) expose discovered tools as an enumerable map; consumers migrating to copilot-sdk lose this.

## Contribution scope

`api.schema.json` ships from the closed `@github/copilot` npm package (the public `github/copilot-cli` repo is just the installer). The server-side RPC handler must live in the internal CLI — external contributors can only touch SDK bindings/wrappers in this repo, not the underlying RPC implementation.

## Proposed solution

Add a session-scoped `tools.list` RPC, mirroring the existing `client.rpc.tools.list()` but reflecting per-session config:

```ts
// in createSessionRpc:
tools: {
list: async (): Promise =>
connection.sendRequest("session.tools.list", { sessionId }),
handlePendingToolCall: /* unchanged */,
}

export interface SessionToolsListResult {
tools: Tool[]; // namespacedName populated for MCP tools (e.g. "playwright/browser_navigate")
}
```

Returns built-ins (post-`excludedTools`/`availableTools` filtering) + MCP-contributed tools. Consistent with existing `session.rpc.{mcp,skills,plugins,extensions}.list()` shape. Parity in Python/Go/.NET per CONTRIBUTING.

A leaner alternative: enrich `McpServer` with `toolCount?: number` and/or `tools?: string[]`. Less consistent with the existing pattern but lighter.

## Related

- #944 — same underlying gap, surfaced as a question.
- #735 — adjacent (manipulating session tool set without teardown).

## Environment

`@github/copilot-sdk@0.3.0`