feat: implement server/discover MCP endpoint

[Vaishnav88sk]

Description

This PR implements the mandatory server/discover endpoint for the Model Context Protocol (MCP) server. This endpoint is required for upfront protocol negotiation and capability discovery by modern MCP clients.

Testing

• Verified local compilation successfully.
• Temporary programmatic tests were written and executed.

Test Execution Logs

[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running io.reshapr.proxy.mcp.McpControllerTest
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.200 s -- in io.reshapr.proxy.mcp.McpControllerTest
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------

Fixes #287

[lbroudoux]

LGTM. Thanks a lot for the contribution!