Fixes the browser proxy crash discussed in:

#43872

That issue includes the original reproduction steps, crash reports, and the NSJSONSerialization -> MacNodeBrowserProxy.makeRequest(...) -> SIGABRT evidence chain.

Greptile Summary

This PR replaces the JSONSerialization-based JSON bridge in MacNodeBrowserProxy with AnyCodable + JSONEncoder/JSONDecoder, fixing crashes that occurred when non-Foundation JSON values (e.g., nested Swift types bridged via __SwiftValue) passed through the proxy. Two targeted regression tests are added.

Key changes:

• ProxyFilePayload is made Encodable and its manual asJSON() helper is removed, replaced by a new ProxyResponse: Encodable wrapper struct.
• Response decoding now uses JSONDecoder().decode(AnyCodable.self, from: data), which handles scalar top-level JSON (e.g., true) that JSONSerialization could produce but the old wrapping pipeline couldn't safely propagate.
• Request body serialization switches from JSONSerialization.data(withJSONObject: body.foundationValue, options: [.fragmentsAllowed]) to JSONEncoder().encode(body), eliminating the __SwiftValue bridging crash on nested objects.
• result.foundationValue bridges back to Any for the unchanged collectProxyPaths / loadProxyFiles logic, keeping that code path working without modification.
• Swift's synthesized Encodable for ProxyResponse uses encodeIfPresent for the optional files property, correctly omitting the key when nil (matching the old behaviour of only setting the key when non-empty).

Confidence Score: 5/5

• This PR is safe to merge — the changes are narrowly scoped, logically correct, and backed by targeted regression tests.
• All changed codepaths have been carefully traced. The Codable round-trip is correct for all JSON value kinds (scalars, objects, arrays, null). The optional-nil omission relies on well-defined Swift behaviour (synthesised Encodable uses encodeIfPresent). The foundationValue bridge preserves the existing collectProxyPaths/loadProxyFiles semantics. No edge-case regressions were found.
• No files require special attention.

_{Last reviewed commit: 1df314d}