Fix #1199: correct ownership transfer in json::detail::r_string move operations (prevent leaks and invalid frees)

[jwolak]

This PR fixes a memory-management issue in json::detail::r_string caused by incorrect move semantics.

Problem
r_string move operations did not safely transfer ownership in all cases, which could lead to:

• memory leaks during move assignment when the destination already owned a buffer,
• incorrect behavior on self-move assignment,
• potential invalid memory handling when move construction fell back to copy-like behavior.

Root Cause

• The move constructor delegated without std::move, effectively using copy-assignment behavior.
• Move assignment did not release the destination’s previously owned buffer.
• No self-assignment/self-move guard was present.

Changes

In json.h:

1. Updated move constructor to use std::move(r) so move assignment is actually invoked.
2. Updated move assignment to:
   • return early on this == &r,
   • release currently owned destination memory before taking ownership from source,
   • transfer pointers/ownership flag and clear source ownership afterward.
3. Added a this == &r guard to copy assignment for consistency/safety.

In test_json.cpp:

1. Added a helper that creates an owned r_string from a copied buffer.
2. Added regression tests for When a crow::json::rvalue is reused and repeatedly assigned from crow::json::load(...), heap usage grows roughly linearly with the number of assignments. #1199 covering:
   • move constructor ownership transfer,
   • move assignment ownership transfer,
   • self-move assignment as a no-op.

Result
- Ownership is now transferred correctly and safely in r_string.
- Leak/double-free risk paths in move-related scenarios are eliminated.
- New regression tests protect against future breakage.

Compatibility / Impact
This is an internal JSON memory-safety fix and does not change the public API. It improves runtime safety and stability.This PR fixes a memory-management issue in json::detail::r_string caused by incorrect move semantics.