Allow Starter creation to use the server-default project

[Icemap]

Summary

• Keep tdc configure project discovery and the explicit/profile project precedence unchanged.
• Allow Starter cluster creation without a resolved project ID by omitting the tidb.cloud/project label and letting TiDB Cloud select the account default.
• Keep an explicitly empty --project-id invalid and prevent empty project labels from reaching the API.
• Update dry-run output, unit tests, black-box tests, live DB coverage, README, AGENTS, and the completed project-resolution spec.

Why

The create resolver treated a missing profile project_id as a local configuration error even though the Starter API supports requests without a project label. This blocked users who intentionally removed the discovered project ID or used credentials and region settings without a configured profile.

The live test also confirms that the server-selected account default can differ from the tidbx_virtual project discovered by tdc configure, so these two defaults are documented and tested separately.

User impact

Project selection now follows this order:

1. explicit non-empty --project-id;
2. the selected profile's project_id;
3. TiDB Cloud's server-selected account default.

Validation

• make test
• make e2e
• make live-e2e-db
• git diff --check

The live lifecycle created, queried, updated, and deleted its uniquely named Starter cluster without a configured or explicit project ID. No tdc-e2e-* cluster remained after the run.