[cy1990]

What

Make a shared MCP connector usable when referenced by its plain (literal) name
by a non-owner viewer, instead of only by the internal mangled key
custom__<parentInstanceId>__<serverName>.

Two coordinated, minimal additions (one commit):

1. Runtime / authorization (service.py):

   • ConnectorService.list_active_mcp_server_names additionally yields the literal
     server name for a shared server (next to the mangled key).
   • ConnectorService.custom_harness_configs materializes the harness spec under the
     literal name too.
   • Both are collision-safe: a viewer's own server always wins the literal name
     (if name not in names/configs); the mangled key is always kept, so existing
     bindings that persist the mangled key keep working unchanged.

2. UI / list surface (custom_mcp.py):

   • expand_custom_instances(shared_view=True) emits, right after each mangled entry,
     an extra alias entry with the same instance_id but mcp_server_name = <plain name>,
     so the plain name appears in listInstances and stays selectable in the chat composer.

Why

Before this change, a shared parent row only surfaced the mangled key to viewers.
Binding an assistant to the plain name therefore:

• got silently dropped by merge_turn_mcp_servers at runtime (the plain name wasn't in
  the authorized set), and
• even the service.py-only half left the composer broken: listInstances
  (expand_custom_instances(shared_view=True)) still returned only the mangled key, so the
  front-end resolveInitialConnectors allowed-set lacked the plain name, dropped the agent's
  plain-name binding from the turn, and MCPToolMiddleware stripped every MCP tool
  (observed as filter: mcp=28 -> 0).

Net effect: a shared connector could not be maintained/bound by plain name from the UI at all.
This PR closes that gap symmetrically across both surfaces (authorization + list).

Independence

This is split out of #1845 (per-user identity injection into connector url/headers). It does
not depend on or touch ${octop.*} placeholders; it concerns shared-connector name
visibility only. #1845 remains orthogonal.

Tests

Adds unit tests for both surfaces (tests/unit/connectors/test_custom_mcp.py):

• authorization/materialisation of the literal alias + own-wins-on-collision,
• expand_custom_instances(shared_view=True) yields the literal alias next to the mangled key.

Full CI-equivalent gate run on the resulting image (toolchain pinned to repo lockfile):

• ruff check src tests / ruff format --check src tests: clean
• mypy src/octop: no issues
• pytest tests/unit/connectors: all pass (incl. the new alias cases)
• dual-layer probe: stock MISS / patched HIT for both symbols

Notes / limits

• listInstances now returns two rows for the same shared server (mangled + literal) with an
  identical display_name; the composer distinguishes them by mcp_server_name (values
  differ). A cosmetic duplicate label; runtime authorization/materialisation stays keyed on the
  literal name to match list_active_mcp_server_names.
• Related issue: [feature] streamable_http 连接器收到 401 时应驱动 MCP OAuth（RFC 9728 + DCR + PKCE），或支持运行期注入当前用户身份 #1827.