Log FlutterJSONMessageCodec decode errors before asserting

[jmagman]

Logging JSON decode errors on assertion will help track down bugs.
flutter/flutter#100891 (comment)

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[jmagman]

Similar to what's already done in encode

engine/shell/platform/darwin/common/framework/Source/FlutterCodecs.mm

Line 82 in ed73d40

NSAssert(encoding, @"Invalid JSON message, encoding failed: %@", error);

[jmagman]

Had to add this flag to allow EXPECT_EXIT.
https://github.com/google/googletest/blob/main/docs/advanced.md#death-test-styles

Otherwise:

[WARNING] ../../third_party/googletest/googletest/src/gtest-death-test.cc:1102:: Death tests use fork(), which is unsafe particularly in a threaded context. For this test, Google Test detected 2 threads. See https://github.com/google/googletest/blob/main/docs/advanced.md#death-tests-and-threads for more explanation and suggested solutions, especially if this is the last message you see before your test times out.
../../flutter/shell/platform/darwin/common/framework/Source/flutter_codecs_unittest.mm:52: Failure
Death test: [codec encode:value]
    Result: died but not with expected error.
  Expected: contains regular expression "failed to convert to UTF8"
Actual msg:
[  DEATH   ] 2024-01-29 18:38:36.256 framework_common_unittests[31544:1799550] *** Assertion failure in -[FlutterJSONMessageCodec encode:], FlutterCodecs.mm:82
[  DEATH   ] objc[31544]: +[NSNumber initialize] may have been in progress in another thread when fork() was called.
[  DEATH   ] objc[31544]: +[NSNumber initialize] may have been in progress in another thread when fork() was called. We cannot safely call it or ignore it in the fork() child process. Crashing instead. Set a breakpoint on objc_initializeAfterForkError to debug.