refactor: serve the ACP v2 session lifecycle without prompts (#1219)

Step 2 of opt-in ACP v2 support (step 1: #1217). Behind
`CLAUDE_AGENT_ACP_EXPERIMENTAL_V2=1`, the v2 surface now serves
`session/new`, `session/list`, `session/resume` (without replay),
`session/close`, `session/delete`, and `session/set_config_option`. ACP
v1 is unchanged.

`capabilities.session` is still not advertised. Under v2, advertising it
commits the agent to the whole baseline, including `session/prompt` and
`replayFrom: start` replay, and those come in later steps. Until then
these methods are served but not advertised.

## Translation

As in step 1, each v2 request becomes the v1 request that
`ClaudeAcpAgent` already serves, and each v1 response or update becomes
its v2 form (`src/v2/session.ts`, `src/v2/session-update.ts`):

- **MCP servers:** v2 requires a `type` on every transport, but the
agent recognizes a v1 stdio server by its *missing* `type`. A v2 `type:
"stdio"` config passed through as is would be silently skipped, so the
tag is dropped. The lists v1 requires (`args`, `env`, `headers`) are
defaulted. A transport v1 can't express is rejected with invalid params
instead of being left out of the session.
- **Config options:** `id` → `configId`, a select group's `group` →
`groupId`. A select value in `session/set_config_option` drops its v2
`type: "id"`, and value types v1 can't express are rejected.
- **Modes:** responses drop `modes`, and `current_mode_update` is
dropped. The agent always lists the mode as the `mode` config option,
and every mode change also reaches the client as a
`config_option_update` or in the `set_config_option` response.
- **Commands:** `available_commands_update` inputs gain `type: "text"`.
- **Notices, session info, usage:** passed through unchanged. v2 needs
no capability for notices, so a v2 client is reported to the agent as
taking them, and advisories arrive as `notice` instead of transcript
messages.
- **Elicitation:** the same in v1 and v2, so it's forwarded. MCP OAuth
at `session/new` works.
- Update kinds that aren't translated yet (messages, tool calls, plans,
extensions) throw, so a gap shows up as an error, not a missing update.
- `replayFrom` is rejected until replayed history can be translated.

Also bumps `@agentclientprotocol/sdk`:
- to 1.6.1, whose router types `closed` as always present, so `serveAcp`
drops its `closed!`;
- to 1.7.0, which type-checks the v2 extensible-union values the adapter
builds, at any depth
([typescript-sdk#268](https://github.com/agentclientprotocol/typescript-sdk/pull/268)).
The v2 translations compile unchanged. The exception is
`createElicitation`, which relays the v1 request: v1 types a property
schema's tag as any string, and an MCP server's schema passes through
unchecked, as on v1. It gets a cast with a comment saying so.

## Testing

- `src/tests/acp-v2.test.ts` drives the router with the SDK's v2 client
app, which validates every response, session update, and elicitation
against the v2 schema. New cases cover session creation with v2 MCP
servers, config options, and commands; rejected MCP transports;
list/close/resume/delete and rejected `replayFrom`; setting the mode
with no `current_mode_update` reaching the client; notice support; MCP
OAuth through URL elicitation; and unit tests for grouped options,
rejected value types, and untranslated updates. Breaking the stdio or
command-input translation makes the tests fail.
- Ran the built binary over stdio against a real Claude Code session: v2
`initialize`, `session/new`, `list`, setting the mode, and `close` all
passed the v2 client's validation, and the 52 real commands arrived with
`type: "text"` inputs. `session/new` took ~0.7 s, the same as over v1.
- `npm run build`, `npm run check`, `npm run test:run`.

package.json

@@ -65,7 +65,7 @@
     "node": ">=22"
   },
   "dependencies": {
-    "@agentclientprotocol/sdk": "1.6.0",
+    "@agentclientprotocol/sdk": "1.7.0",
     "@anthropic-ai/claude-agent-sdk": "0.3.286",
     "diff": "9.0.0",
     "zod": "4.6.5"