fix(filesystem): preserve literal \$ in edit_file newText (closes #4157)

[adityachilka1]

Fixes #4157.

This is the exact fix the issue reporter proposed (and is consistent with the
bug root-cause in the issue body).

The bug

src/filesystem/lib.ts passes normalizedNew as String.prototype.replace's
replacement argument. When that argument is a string, JS interprets these
tokens in it:

Token	Interpreted as
$$	a literal $
$&	the matched substring
`$``	the portion before match
$'	the portion after match
$n	nth capture (regex only)

So when a caller does edit_file with newText: "$100 USD", the $1 token
gets eaten (replaced with the content of capture group 1, which is undefined
for a plain-string search, so it's dropped).

The fix

Use the callback form (() => normalizedNew). MDN explicitly documents
this as the way to disable replacement-pattern interpretation:

If replacement is a function, the matched substring is replaced with the
return value of the function.

One-line diff:

-      modifiedContent = modifiedContent.replace(normalizedOld, normalizedNew);
+      modifiedContent = modifiedContent.replace(normalizedOld, () => normalizedNew);

Tests

Two regression tests added to src/filesystem/__tests__/lib.test.ts:

1. $$ preservation (literal $)
2. $&, $``, $'` preservation (replacement-pattern tokens)

Both tests assert that the exact input string is written verbatim. Without
the fix, both tests fail.

Verification

cd src/filesystem && npm test

-> 148 / 148 tests pass across 7 test files (the 2 new tests are in there).

No behavioral regression

The callback always returns a constant ({n normalizedNew {n unbound}), so for
inputs that don't contain $, the behavior is identical to the string-arg form.
The only change is that $ characters now survive the replace.

Closes #4157.