fix(mcp): merge multiple JSON objects in MCP tool output

[twishabansal]

Context

When an MCP tool returns multiple JSON objects (e.g., database rows) as separate text chunks, tool_invoke
was concatenating them directly (e.g., {}{}), resulting in invalid JSON.

Changes

• Updated tool_invoke in all MCP protocol versions (v20241105, v20250326, v20250618).
• Added a heuristic: if multiple chunks all look like complete JSON objects (start/end with {}), they are now merged into a JSON list [{},{}].
• Added regression tests to cover this case while maintaining existing behavior for standard text streams.

Fixes #503