Fix `BTInstance` not registered with debugger on BTPlayer.set_bt_instance() #793

Job	Run time
cache-env	1s
🧪 Unit tests	3m 9s
⚙️ Static checks	7s
🍎 macOS / Editor (x86_64, release)	3m 11s
🔌 GDExtension / 🐧 Linux (x86_64, release)	32s
🍎 macOS / Editor (arm64, release)	3m 29s
🔌 GDExtension / 🐧 Linux (x86_64, debug)	29s
🪟 Windows / Editor (x86_64, release)	18m 33s
🐧 Linux / Editor (x86_64, release)	14m 45s
🔌 GDExtension / 🪟 Windows (x86_64, release)	2m 29s
🔌 GDExtension / 🪟 Windows (x86_64, debug)	1m 43s
🐧 Linux / Template (x86_64, release)	9m 56s
🔌 GDExtension / 🍎 macOS (universal, release)	1m 4s
🐧 Linux / Editor .NET (x86_64, release)	17m 45s
🔌 GDExtension / 🍎 macOS (universal, debug)	1m 1s
🔌 GDExtension / 📦 Package extension	29s
🍎 macOS / Make macOS Bundles (tmp-macos, false, true)	37s
🪟 Windows / .NET assembly for ${{ matrix.opts.arch }}	0s
	1h 19m 20s

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