feat: add custom tools support via add_tool() and custom_tools parameter

[nachogarcia]

Describe your changes

Adds a public API for registering custom MCP tools that don't map to FastAPI endpoints. This enables use cases like wrapping SSE streams, returning instructions, or bridging protocol gaps — without monkey-patching internal methods.

New API

from fastapi_mcp import FastApiMCP, HTTPRequestInfo
import mcp.types as types

# Option A: at construction time
mcp = FastApiMCP(
    app,
    custom_tools=[(my_tool, my_handler)],
)

# Option B: after construction, before mount
mcp.add_tool(my_tool, my_handler)

Handler signature:

async def my_handler(name, arguments, http_request_info, server):
    return [types.TextContent(type="text", text="response")]

Handlers receive the same HTTPRequestInfo (auth headers, cookies, etc.) that auto-generated tools get, plus the MCP Server instance for access to request_context.

What changed

• fastapi_mcp/server.py: Added _custom_handlers dict, custom_tools constructor param, add_tool() method, and a dispatch branch in handle_call_tool that routes to custom handlers before falling through to _execute_api_tool
• fastapi_mcp/__init__.py: Exported HTTPRequestInfo so consumers can type-hint handlers
• tests/test_custom_tools.py: 7 new tests covering listing, calling, request info forwarding, server instance access, constructor param, OpenAPI tool coexistence, and error handling

~30 lines of new production code. No breaking changes — all existing behavior is preserved.

Issue ticket number and link (if applicable)

Closes #75

Checklist before requesting a review

• Added relevant tests
• Run ruff & mypy
• All tests pass