Silence a GCC dangling-reference warning in controller parsing

[kahrendt]

Bind the supported_commands array to a named local before iterating it
in process_server_state_controller(), so GCC stops emitting
-Wdangling-reference there.

Why

The warning fires on every ESP-IDF build of the component (including via
ESPHome), and is present in released v0.7.1 and on current main:

src/protocol.cpp:554:73: warning: possibly dangling reference to a temporary [-Wdangling-reference]
  554 |              controller_object["supported_commands"].as<JsonArrayConst>()) {
      |                                                                         ^
src/protocol.cpp:554:52: note: 'MemberProxy<JsonObject, RamString>' temporary created here

The range initializer chained .as<JsonArrayConst>() off the
MemberProxy temporary returned by operator[]. Only the final
temporary in such a chain gets lifetime extension in C++20, so the proxy
is destroyed at the end of the full-expression while the loop body is
still running.

It was benign. JsonArrayConst stores only a const ArrayData* and
a const ResourceManager*, both pointing into the JsonDocument's
memory pool, and JsonArrayConstIterator holds an ArrayData::iterator
plus that same resource pointer. Neither the array nor its iterators
retain anything that points into the dead proxy, so the destroyed
temporary was never read. GCC's heuristic cannot see through the proxy
type, though, so this fixes the code rather than suppressing the
diagnostic.

How

-        for (JsonVariantConst command_var :
-             controller_object["supported_commands"].as<JsonArrayConst>()) {
+        JsonArrayConst commands_array =
+            controller_object["supported_commands"].as<JsonArrayConst>();
+        for (JsonVariantConst command_var : commands_array) {

This is also what the rest of the file already does. The five other
array loops (active_roles, artwork channels, visualizer types, and
the roles lists in stream/end and stream/clear) each name a local
first; the loop at line 264 ranges over a function parameter. This was
the only chained range initializer left, so the change makes the file
consistent rather than introducing a new idiom.

No behavior change.

Testing

macOS builds with clang, which does not implement -Wdangling-reference
and so never reproduced this. Verified with Homebrew g++-15 driven at
the host build's exact flags and include set:

• Warning reproduces verbatim before the change, and is gone after it.
• Swept every file in src/ and src/host/ with
  -Wall -Wextra -Wdangling-reference -fsyntax-only: no
  dangling-reference warnings remain anywhere.
• Host clang build rebuilds clean; ctest 123/123 pass.
• clang-format --dry-run -Werror src/protocol.cpp clean.

[Copilot]

Pull request overview

This PR addresses a GCC -Wdangling-reference warning in process_server_state_controller() by avoiding a chained temporary in a range-based for loop, aligning this loop with the existing local-binding pattern used elsewhere in src/protocol.cpp.

Changes:

• Bind controller_object["supported_commands"].as<JsonArrayConst>() to a named local (commands_array) before iterating.
• Keep parsing behavior unchanged while removing a noisy compiler warning on ESP-IDF/GCC builds.

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