fix: dupe ca string in .npmrc to prevent use-after-free (#25563)

## Summary

- Fix use-after-free bug when parsing `ca` option from `.npmrc`
- The `ca` string was being stored directly from the parser's arena
without duplication
- Since the parser arena is freed at the end of `loadNpmrc`, this
created a dangling pointer

## The Bug

In `src/ini.zig`, the `ca` string wasn't being duplicated like all other
string properties:

```zig
// Lines 983-986 explicitly warn about this:
// Need to be very, very careful here with strings.
// They are allocated in the Parser's arena, which of course gets
// deinitialized at the end of the scope.
// We need to dupe all strings

// Line 981: Parser arena is freed here
defer parser.deinit();

// Line 1016-1020: THE BUG - string not duped!
if (out.asProperty("ca")) |query| {
    if (query.expr.asUtf8StringLiteral()) |str| {
        install.ca = .{
            .str = str,  // ← Dangling pointer after parser.deinit()!
        };
```

All other string properties in the same function correctly duplicate:
- `registry` (line 996): `try allocator.dupe(u8, str)`
- `cache` (line 1002): `try allocator.dupe(u8, str)`
- `cafile` (line 1037): `asStringCloned(allocator)`
- `ca` array items (line 1026): `asStringCloned(allocator)`

## User Impact

When a user has `ca=<certificate>` in their `.npmrc` file:
1. The certificate string is parsed and stored
2. The parser arena is freed
3. `install.ca.str` becomes a dangling pointer
4. Later TLS/SSL operations access freed memory
5. Could cause crashes, undefined behavior, or security issues

## Test plan

- Code inspection confirms this matches the pattern used for all other
string properties
- The fix adds `try allocator.dupe(u8, str)` to match `cache`,
`registry`, etc.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

src/ini.zig

@@ -1016,7 +1016,7 @@ pub fn loadNpmrc(
     if (out.asProperty("ca")) |query| {
         if (query.expr.asUtf8StringLiteral()) |str| {
             install.ca = .{
-                .str = str,
+                .str = try allocator.dupe(u8, str),
             };
         } else if (query.expr.isArray()) {
             const arr = query.expr.data.e_array;