agent_servers: Move the legacy ACP adapter into v1 (#65607)

- Move `AcpConnection`, its defaults, the v1 builder/capabilities/wire
handlers, legacy session/config/MCP operations and session-list
implementation into `acp/v1.rs` alongside `acp/v2.rs`.
- Keep foreground routing/context, shared session/config data, v2 client
ingress, canonical permission/elicitation application, shared
terminal-auth construction and error/lifecycle helpers in the
root/shared modules.
- Keep only the v1 config-setting wire method on the shared config data
in `v1.rs`; the shared constructor/state stays in the root. The existing
mixed session record is not redesigned in this extraction.
- Preserve public paths through root reexports (`AcpConnection`,
`AcpSessionList`, `connect`, the Gemini terminal-auth constant and
conditional `test_support`). Consumer call sites are unchanged. The
shared-runtime test imports the moved private v1 driver directly.
- Keep compatibility/ingress tests alongside the legacy harness they
exercise; v2 connection/auth tests stay in `v2.rs`. Test names gain the
v1 module prefix but their bodies and scheduler behavior are unchanged.

## Review

This is a large source relocation, not thousands of new implementation
lines. For copy-aware review:

```sh
git diff -B -M origin/main...HEAD -- crates/agent_servers/src/acp.rs crates/agent_servers/src/acp/v1.rs crates/agent_servers/src/acp/runtime.rs
```

Git recognizes `v1.rs` as a 93%-similar copy of the old adapter. Review
the retained root boundary and the small import/visibility changes
rather than re-reviewing unchanged test bodies.

Release Notes:

- N/A

crates/agent_servers/src/acp/runtime.rs

@@ -191,7 +191,7 @@ pub(super) fn watch_process(
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::acp::connect_client_future;
+    use crate::acp::v1::connect_client_future;
     use acp_thread::{ElicitationStatus, ElicitationStore};
     use agent_client_protocol::schema::v2 as acp;
     use collections::HashMap;