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

[dhruba-datta]

Fixes #4157.

Root cause

applyFileEdits (in src/filesystem/lib.ts) invoked
String.prototype.replace(searchString, replacementString) where the
second argument is interpreted as a replacement-string pattern,
not a literal string. This silently corrupted content containing
literal $ characters:

Pattern in newText	Replaced by
$$	a single $
$&	the matched substring
$`	the substring preceding the match
$'	the substring following the match
$<name>	(regex named-group syntax)

Particularly hit JS/TS code with $variables, currency amounts, regex
source, and shell scripts. The fallback line-by-line path (via splice
/ join) was not affected.

Fix

Pass the replacement via the callback form:

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

The function's return value is used verbatim, which bypasses the
pattern interpreter while preserving "replace first occurrence only"
semantics. One-line change to the production code.

Test coverage

Added a regression test in __tests__/lib.test.ts covering all five
pattern cases ($$, $&, $`, $', $<name>). The new test runs
alongside the existing applyFileEdits test block and matches the
file's existing style.

Full vitest suite: 147 passed, 0 failed locally after the change.

Impact

@modelcontextprotocol/server-filesystem has ~297k weekly downloads
per the issue, and the corruption is silent — edit_file returns
success and a clean-looking diff while the on-disk content differs
from intent.