bug: Restore GCP metadata server support

[feloy]

User Story

As a user running a Google Cloud SDK in an OpenShell sandbox, I want the GCP metadata endpoint to be available, so that the SDK can discover project and service-account information through its standard metadata flow.

Problem Statement

The GCP metadata server/emulator is no longer available in current sandbox runtimes. The google-cloud provider example requires the gcp-metadata platform adapter, but importing, updating, or attaching that profile fails with required platform adapter 'gcp-metadata' is unavailable in this build. Workloads that rely on GCP SDK metadata discovery therefore cannot use the provider as documented.

The implementation files crates/openshell-sandbox/src/google_cloud_metadata.rs and crates/openshell-sandbox/src/metadata_server.rs were removed in PR #2942 during the runtime split. The removal was raised in PR #3775, where maintainers requested a bug to track adding the capability back.

Impact / Why This Matters

Users cannot run GCP SDKs that expect the standard metadata service inside OpenShell, including flows that obtain project and service-account details through metadata discovery. They must replace that flow with manually configured values or credentials, which is not equivalent for workloads designed to use metadata-based authentication and reduces portability across GCP environments.

Acceptance Criteria

• OpenShell provides the documented GCP metadata endpoint to sandbox workloads on supported runtimes.
• A GCP SDK can perform metadata discovery, including retrieving the expected project/service-account metadata, using the standard metadata request flow.
• Automated coverage verifies the metadata behavior and prevents an unsupported gcp-metadata profile from being presented as usable.

Reproduction Steps

1. Use a current OpenShell build containing the runtime split from PR feat(isolation): implement the RFC 0012 sandbox architecture #2942.
2. Import or attach the google-cloud provider profile, which declares required_platform_adapter: gcp-metadata.
3. Observe the required platform adapter 'gcp-metadata' is unavailable in this build error. In builds/configurations where the profile can be attached, run a GCP SDK that queries http://metadata.google.internal/computeMetadata/v1/ with the Metadata-Flavor: Google header; metadata discovery is unavailable.

Environment

• OpenShell: Current build after PR feat(isolation): implement the RFC 0012 sandbox architecture #2942 (exact version not provided)
• OS: Not specified
• Runtime, deployment, or integration: OpenShell sandbox runtime; affected compute driver not specified

Logs

required platform adapter 'gcp-metadata' is unavailable in this build