fix: Validate string format of SDK key

[jsonbailey]

Note

Validate SDK key format, ignore invalid keys during config, emit error log on startup if invalid, and update CommonSdk/InternalSdk versions.

• Configuration:
  • Validate SDK key via ValidationUtils.IsValidSdkKeyFormat; ConfigurationBuilder now ignores invalid keys in constructor, copy constructor, and SdkKey().
• Client runtime:
  • Log an error at startup if _configuration.SdkKey is null while not offline in LdClient.
• Tests:
  • Update test to use a non-empty SDK key in LdClientListenersTest.
• Dependencies:
  • Bump LaunchDarkly.CommonSdk to 7.1.1 and LaunchDarkly.InternalSdk to 3.5.5.}

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