MCP Server Base64 encode/decode mismatch for Resources

[evalstate]

Describe the bug

In src/mcp/server/lowlevel/server.py, binary data is encoded using base64.urlsafe_b64encode(), which replaces + with - and / with _ to make it URL-safe.
However, when the client tries to validate this data with BlobResourceContents, the validator expects standard base64 encoding (with + and /).
This causes validation failures with "Invalid base64" errors when processing larger binary files that contain these special characters.

Before Fix:

# Create server
server = FastMCP("Base64 Test")

# Add resource directly
binary_resource = FileResource(
    uri=AnyUrl("resource://test/simple.bin"),
    path=Path("sample.pdf").absolute(),
    name="Test PDF",
    mime_type="application/pdf",
)

server.add_resource(binary_resource)

if __name__ == "__main__":
    server.run()

Use FastMCP to return a binary resource (e.g. a PDF file), and inspect the contents.

After Fix:

Expected behavior
That binary content is returned as correctly base64 content rather than an error message.

Additional context
PR incoming with automated test.

[evalstate]

Thanks, will unblock this test : https://github.com/evalstate/fast-agent/blob/8891d00e5d069eaddebadf8bbf1d775426e3ef1c/tests/e2e/prompts-resources/test_resources.py#L4-L34