mcp: support '/' and '___' prefixes; add streamable affinity; tests

[ljluestc]

Ⅰ. Describe what this PR did

• mcp-router: Support both “server___tool” and “server/tool” prefixes when routing tools/call, ensuring correct backend selection regardless of client/tooling prefix style.
• Nacos MCP watcher: Add consistent-hash (source IP) load balancing for Streamable HTTP transport (in addition to SSE) to improve backend affinity and prevent intermittent tool denial.
• Add unit tests:
  • mcp-router: prefix parsing for both separators.
  • nacos watcher: DestinationRule affinity for SSE and Streamable.

Ⅱ. Does this pull request fix one issue?

• fixes 使用nacos+higress 将存量API转换为MCP访问 /mcp/{mcp-server-name}/sse 能正常连接，但是调用tool时有一定几率出现 -32602 Tool not allowed 报错 || Use nacos+higress to convert the existing API to MCP access /mcp/{mcp-server-name}/sse to connect normally, but there is a certain chance of a tool when calling -32602 Tool not allowed Report an error #2856

Ⅲ. Why don't you add test cases (unit test/integration test)?

• Added unit tests:
  • plugins/wasm-go/mcp-filters/mcp-router/main_test.go: validates parsing for “server___tool” and “server/tool”.
  • registry/nacos/mcpserver/watcher_test.go: asserts consistent-hash policy for SSE and Streamable protocols.

Ⅳ. Describe how to verify it

• Functional:
  • Register an MCP server via Nacos with SSE or Streamable HTTP transport.
  • Connect SSE at /mcp/{mcp-server-name}/sse (or streamable endpoint).
  • Invoke tools using both name styles:
    • params.name: "{server}___{tool}"
    • params.name: "{server}/{tool}"
  • Expect no -32602 Tool not allowed; tool should be accepted consistently.
• Observability:
  • Confirm DestinationRule for the generated service uses ConsistentHash for SSE and Streamable.
  • With jsonrpc-converter enabled, verify headers x-envoy-mcp-tool-name and x-envoy-jsonrpc-method reflect the tool routed without the server prefix.
• Regression:
  • Requests without a prefix remain untouched and route as before.

Ⅴ. Special notes for reviews

• Backward compatible: legacy “server___tool” continues to work; “server/tool” newly supported.
• Affinity note: Source-IP hashing is best-effort behind shared NAT; if stronger stickiness is needed, consider header-based consistent hash (e.g., x-mcp-session-id) in a follow-up change.
• Scope-limited to MCP filters and Nacos MCP watcher; no changes to unrelated paths.

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