Zed nightly: 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:

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