[BUG]: Spacing issue in lists

[adhiamboperes]

Describe the bug

Related to #6430 and #6431:

After a list, there is no spacing between the last list item and the next item, whether a non-list item or an outer list item.

Some sentences are have the appearance of missing spacing before/after, making them run into each other after a period(.) and look like links. For this specific bug #6432 fixed it in worked examples and it appears that some of the list tags were previously getting stripped. The same fix applied to worked examples should be applied to the broader lists.

Steps To Reproduce

Open a study guide with lists.
See that subsequent lists have no spacing in between
Also notice that some text have no spacing in between, showing up as links.

Expected Behavior

Lists should display corrcetly, with proper spacing, e.g.:

Screenshots/Videos

What device/emulator are you using?

No response

Which Android version is your device/emulator running?

Android 10

Which version of the Oppia Android app are you using?

0.18-dev

Additional Context

No response

[adhiamboperes]

@Neer-rn, this is out of scope for GSoC(occurs even with study guides and worked examples feature flags off), but is related.

Could you please update the issue with an explanation of the bug you discovered in worked examples, and how to fix it, so that someone else can pick pick it up? Once done, please add the gfi label.

[Neer-rn]

I investigated this while working on #6432. Some list tags in the lesson HTML have extra attributes, for example <ul xmlns="...">. Earlier, the parser only matched exact tags such as <ul> and <li>. Because of this, some list tags were not handled correctly. This could join the text together and sometimes make it look like a link.

#6432 fixes this by matching list tags even when they have extra attributes. There is also another spacing problem in LiTagHandler. When a ul or ol list ends, the handler adds only one newline. Because of this, there is no empty line between the last list item and the next outer list item or paragraph. To fix the remaining issue, LiTagHandler should add enough newlines after a closing list tag to create one blank line. Tests should also be added for:

• A nested list followed by another outer list item.
• A list followed by normal text or a paragraph.

[AryanSingh1938]

@Neer-rn
i would love to solve this issue, could you please assign it to me...

[Neer-rn]

@AryanSingh1938 Please explain the approach you plan to take to solve this issue. If the approach looks good, I can assign the issue to you.

[AryanSingh1938]

the core logic is only use a single newline between list items, and add a double newline only when the outermost list closes.
-> I will replace strict string matching for list tags with regex matching to support HTML tags containing XML namespaces or attributes .
-> i will Handle whitespaces and paragraph boundaries explicitly within "LiTagHandler" by injecting proper newline separations between adjacent list items and around nested blocks,

Moreover i have fix this issue to some extent locally in my machine...Here are screenshots:

If there is some issue please tell me i will modify it further but i think it solves all the problem...please assign me the issue or tell me where i should improve.