fix: eliminate data races in Debugger and ServiceKey

[holyfuchs]

Debugger.listener was written in Start() and read in Stop() concurrently with no synchronization. Added a mutex to guard all accesses and a stopped flag so if Stop() runs before Start() sets the listener, Start() detects it and closes the listener immediately instead of starting the serve loop.

defaultConfig is a package-level variable whose ServiceKey.PrivateKey is a shared pointer across all tests. The onflow/crypto library lazily initializes the public key inside the private key struct without synchronization, causing a race when parallel tests both trigger PublicKey(). Fixed by eagerly initializing and storing the public key at key-generation time, removing the lazy path entirely.

[github-actions]

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