Add ghostty_config_get tests (#10891)

I mostly did this to familiarize myself with the codebase and figured it
doesn't hurt to cover this with tests if more added logic in this area,
despite this logic receiving indirect coverage elsewhere. [Here's my
related
proposal](https://github.com/ghostty-org/ghostty/discussions/10807).

I gave more thought around how to expose some of these config values and
their metadata in the C api to eventually drive a settings UI and was
hoping for feedback before I proceed.

The cleanest path forward feels like annotating config values with
formal metadata around things like: supported platforms, whether or not
a restart is required, presentation metadata like grouping + ordering,
tolerated ranges for values, possible enum values, etc. My intent is
that Swift & other consumers can enumerate potential settings values
with metadata such as to drive the UI from the metadata.

---

AI Disclosure: I used Codex 5.3 to help me understand how the config
subsystem in zig is exposed to Swift via the C API. Codex wrote these
tests; but we brainstormed on a pragmatic coverage balance and I
understand how the tests work.

src/config/CApi.zig

@@ -144,3 +144,101 @@ export fn ghostty_config_open_path() c.String {
 const Diagnostic = extern struct {
     message: [*:0]const u8 = "",
 };
+
+test "ghostty_config_get: bool" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var cfg = try Config.default(alloc);
+    defer cfg.deinit();
+    cfg.maximize = true;
+
+    var out = false;
+    const key = "maximize";
+    try testing.expect(ghostty_config_get(&cfg, &out, key, key.len));
+    try testing.expect(out);
+}
+
+test "ghostty_config_get: enum" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var cfg = try Config.default(alloc);
+    defer cfg.deinit();
+    cfg.@"window-theme" = .dark;
+
+    var out: [*:0]const u8 = undefined;
+    const key = "window-theme";
+    try testing.expect(ghostty_config_get(&cfg, @ptrCast(&out), key, key.len));
+    const str = std.mem.sliceTo(out, 0);
+    try testing.expectEqualStrings("dark", str);
+}
+
+test "ghostty_config_get: optional null returns false" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var cfg = try Config.default(alloc);
+    defer cfg.deinit();
+    cfg.@"unfocused-split-fill" = null;
+
+    var out: Config.Color.C = undefined;
+    const key = "unfocused-split-fill";
+    try testing.expect(!ghostty_config_get(&cfg, @ptrCast(&out), key, key.len));
+}
+
+test "ghostty_config_get: unknown key returns false" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var cfg = try Config.default(alloc);
+    defer cfg.deinit();
+
+    var out = false;
+    const key = "not-a-real-key";
+    try testing.expect(!ghostty_config_get(&cfg, &out, key, key.len));
+}
+
+test "ghostty_config_get: optional string null returns true" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var cfg = try Config.default(alloc);
+    defer cfg.deinit();
+    cfg.title = null;
+
+    var out: ?[*:0]const u8 = undefined;
+    const key = "title";
+    try testing.expect(ghostty_config_get(&cfg, @ptrCast(&out), key, key.len));
+    try testing.expect(out == null);
+}
+
+test "ghostty_config_get: float" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var cfg = try Config.default(alloc);
+    defer cfg.deinit();
+    cfg.@"background-opacity" = 0.42;
+
+    var out: f64 = 0;
+    const key = "background-opacity";
+    try testing.expect(ghostty_config_get(&cfg, &out, key, key.len));
+    try testing.expectApproxEqAbs(@as(f64, 0.42), out, 0.000001);
+}
+
+test "ghostty_config_get: struct cval conversion" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var cfg = try Config.default(alloc);
+    defer cfg.deinit();
+    cfg.background = .{ .r = 12, .g = 34, .b = 56 };
+
+    var out: Config.Color.C = undefined;
+    const key = "background";
+    try testing.expect(ghostty_config_get(&cfg, @ptrCast(&out), key, key.len));
+    try testing.expectEqual(@as(u8, 12), out.r);
+    try testing.expectEqual(@as(u8, 34), out.g);
+    try testing.expectEqual(@as(u8, 56), out.b);
+}