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fix: treat single-element brace-init as copy/move instead of wrapping in array (#5074) (#5090)

* fix: treat single-element brace-init as copy/move

When passing a json value using brace initialization with a single element
(e.g., `json j{someObj}` or `foo({someJson})`), C++ always prefers the
initializer_list constructor over the copy/move constructor. This caused
the value to be unexpectedly wrapped in a single-element array.

This bug was previously compiler-dependent (GCC wrapped, Clang did not),
but Clang 20 started matching GCC behavior, making it a universal issue.

Fix: In the initializer_list constructor, when type deduction is enabled
and the list has exactly one element, copy/move it directly instead of
creating a single-element array.

Before:
  json obj = {{"key", 1}};
  json j{obj};   // -> [{"key":1}]  (wrong: array)
  foo({obj});    // -> [{"key":1}]  (wrong: array)

After:
  json j{obj};   // -> {"key":1}   (correct: copy)
  foo({obj});    // -> {"key":1}   (correct: copy)

To explicitly create a single-element array, use json::array({value}).

Fixes the issue #5074

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* fix: regenerate amalgamated single_include/nlohmann/json.hpp

- Add missing comment from include/nlohmann/json.hpp explaining the
  single-element brace-init fix (issue #5074)
- Fix extra 4-space indentation in embedded json_fwd.hpp section

Regenerated by running: make amalgamate

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* Revert brace-init semantics change and fix amalgamation

The single-element brace-init change was a breaking change that cannot be accepted upstream. Reverted all related source, test, and doc changes, then regenerated single_include with correct indentation to pass the amalgamation CI check.

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* Fix: add JSON_BRACE_INIT_COPY_SEMANTICS opt-in macro for issue #5074

Single-element brace initialization wrapping in an array... (continued)

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