28924 [Linux, Embedder] Add engine restart hooks (cla: yes, platform-linux, embedder)

28937 Cherry-pick #28846 and #28743 into flutter-2.6-candidate.11 (platform-ios, platform-android, cla: yes, needs tests, embedder)

29117 updated the docs for platform_message_callback (cla: yes, embedder)

29408 Embedder VsyncWaiter must schedule a frame for the right time (cla: yes, embedder)

platform-linux - 14 pull request(s)

27757 Rename fl_get_length to fl_value_get_length (cla: yes, waiting for tree to go green, platform-linux)

28204 Fix stack-buffer-overflow in FlValueTest.FloatList found by Asan (cla: yes, platform-linux)