Fix unstable trailing comment on a nested parenthesized assignment - #19893

Description

Fixes the remaining case in #19271.

#19276 made (a = c /* comment */); stable by attaching the trailing comment to the ExpressionStatement. The nested form was left out and is still unstable on main:

// Input
assigned = (a = c /* comment */);

// First format
assigned = a = c /* comment */;

// Second format
assigned = a = c; /* comment */

The second format is already the fixpoint, so this makes the first one produce it too.

handleParenthesizedExpressionTrailingComment does fire here — it just attaches the comment inward to c. Once the parentheses are dropped the comment is at the end of the statement, and the next run attaches it to the statement instead, which is where the instability comes from.

Attaching to the statement on the first pass needs the statement, and the handler only had enclosingNode. For a chain like x = y = (a = c /* comment */) that is the middle assignment, not the outermost, so walking up is the only way to reach it. decorateComment already builds the ancestor chain and threads it through the recursion for canAttachComment, it just wasn't returned, so this returns it and walks up through the right hand side of each enclosing assignment.

Scope is deliberately narrow, since #19273 was reverted once already. Only assignment chains that end in an ExpressionStatement are affected — everywhere else the parentheses are kept, so nothing was unstable to begin with:

const v = (a = c /* comment */);        // unchanged, parens kept
function f() { return (a = c /* c */); } // unchanged, parens kept
assigned = (a, b /* comment */);         // unchanged, sequences unaffected

Two related variants are still unstable, both unchanged by this PR and both failing identically on main:

assigned = (a = (c /* comment */));      // comment parenthesized around `c`
assigned = (a = c /* comment */), other; // inside a sequence

They need a wider change than this one and I'd rather not fold them in here.

parenthesized-trailing-comment-unstable.js existed only for this case, with a TODO to move it back once stable, so it's removed along with its failed-format-tests.js entry and the case is enabled in parenthesized-trailing-comment.js.

Full tests/format run has the same failures before and after.

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