Declare Google Meet realtime secret inputs

Google Meet realtime provider API keys could be configured as SecretRefs, but the bundled manifest did not declare realtime.providers.*.apiKey as a secret input. Secret-backed values such as exec:onepassword refs were not collected before plugin register and realtime paths, blocking join/create flows.

Declared the narrow realtime.providers.*.apiKey secret-input contract in the Google Meet plugin manifest, updated SecretRef documentation and credential-matrix metadata, and added bundled-manifest runtime collection coverage for Google and OpenAI provider entries.

• Restores Google Meet join/create and realtime attachment paths when provider credentials are stored through SecretRef-backed auth.
• Keeps secret resolution centralized in the existing manifest-declared collector instead of adding plugin-specific runtime logic.
• Limits collection to the intended provider API-key path, reducing the risk of over-collecting unrelated Google Meet plugin config.

• Added runtime secret-collector regression coverage for bundled Google Meet manifest metadata and both Google/OpenAI API-key SecretRefs.
• Validated the package-manifest contract, target registry coverage, oxlint, manifest JSON parsing, and repository diff checks from the PR workflow.
• Real behavior proof used a Node 22 runtime harness to collect Google Meet realtime provider assignments with no warnings.