[Feature][Java] Move MCP resource discovery to runtime

[yanand0909]

Linked issue: 608

Purpose of change

MCP tools and prompts were previously discovered at compile time inside AgentPlan.extractJavaMCPServer(): an MCPServer connection was opened, listTools()/listPrompts() were called over the network, results were serialized into the plan, and the connection was immediately closed. This had three problems:

1. Build-time server dependency — AgentPlan construction failed if the MCP server was unreachable, even though the agent wouldn't run until later.
2. Static capabilities — Tools and prompts were frozen at compile time; any server-side changes required recompilation.
3. No connection reuse — The connection opened for discovery was closed immediately; a new one was established later for actual tool calls.

This PR implements Part 1: Runtime Discovery from discussion #543. MCP tool and prompt discovery is moved from AgentPlan construction to ActionExecutionOperator.open():

Tests

• AgentPlanDeclareMCPServerTest — Updated to assert the new compile-time contract: TOOL and PROMPT providers are absent from AgentPlan providers (deferred to runtime), only MCP_SERVER is registered. Tool/prompt retrieval tests now instantiate the MCPServer directly from the plan's provider, simulating what JavaMCPResourceDiscovery does at operator startup.
• JavaMCPResourceDiscoveryTest (new) — Six unit tests covering JavaMCPResourceDiscovery with stub objects (no live server required):
  • Happy path: tools and prompts are discovered and placed in ResourceCache
  • supportsPrompts() = false: prompt discovery is skipped
  • Non-JavaResourceProvider entries are ignored

API

No public API changes

Documentation

• doc-needed
• doc-not-needed
• doc-included

[wenjin272]

Thanks for your contribution, @yanand0909. Due to the upcoming traditional Chinese holiday, my review may be slightly delayed.