fix: Raise resource not found error

[vincent0426]

Introduce resource not found error handling following the MCP specification.

This change ensures that when a resource is not found, the server returns error code -32002 (RESOURCE_NOT_FOUND) as specified in the Resources Error Handling.

Concern:
There is currently no explicit error handling for FastMCP exceptions in the low-level server. I'm not sure if we want to wrap fastmcp exceptions under McpError to raise the correct error code which avoid changing the low-level server code, otherwise, sdk might need to create a new exception type to handle non-connection error?

Motivation and Context

Closes #1579

Previously, when a resource was not found, the server returned error code 0 (generic error) instead of the spec-compliant -32002 (RESOURCE_NOT_FOUND).

Root Cause:

• FastMCP raises ValueError when a resource is not found
• low-level server's exception handler was not properly recognizing these exceptions
• Exceptions without an error attribute were converted to ErrorData(code=0, ...)

python-sdk/src/mcp/server/lowlevel/server.py

Lines 705 to 708 in 3390e49

	except Exception as err:
	if raise_exceptions:
	raise err
	response = types.ErrorData(code=0, message=str(err), data=None)

Solution:

• Added ResourceError exception class with an error attribute containing ErrorData with the appropriate error code
• Updated ResourceManager to raise ResourceError with RESOURCE_NOT_FOUND code for unknown resources

How Has This Been Tested?

• unit test
• Reproduced and verified the fix for the original issue

data: {"jsonrpc":"2.0","id":7,"error":{"code":-32002,"message":"Unknown resource: https://example.com/does-not-exist"}} 

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[maxisbey]

Related: modelcontextprotocol/modelcontextprotocol#1545

Due to ongoing discussions around exactly what errors to use here we should wait on that before merging any fixes for this.