Fix use-before-initialize (NPE) in TextDocumentState

[sungshik]

The UI tests on Ubuntu have become flaky (#1034 and #1037). This PR fixes a bug in the constructor of TextDocumentState to remove one cause of the flakiness.

Problem

Before this PR, the constructor of TextDocumentState:

1. Creates an initial Update object. The constructor of Update:
   1. Submits the parse call for the initial content to the worker pool.
   2. After parsing, stores the tree in an AtomicReference field of TextDocumentState.
2. Initializes an AtomicReference object to store a tree after parsing.

Due to scheduling, it could happen that step 1.ii was executed before step 2. In that case, an NPE occurs, as an attempt is made to set the AtomicReference with the tree, but it hasn't been initialized yet. This NPE is swallowed inside a completion stage, though, so only the fallout (of not having stored the tree) is observable -- this is what happened in the UI tests.

Solution

This PR:

• Makes sure that the AtomicReference object is initialized before use. This addresses the root cause of the issue.
• Makes sure that the post-parsing logic is executed in the worker pool (instead of the request pool). This doesn't address the root cause of the issue, but it is adjacent to it.
• Improves exception reporting to make future debugging easier.

Related PRs (subsumed by this PR):

• Fix that didn't work: Run UI test on Ubuntu 22.04 again #1051
• Debugging: Use slightly longer delays for Ubuntu tests #1040

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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