Shared Multi Step Form for Object Storage Connectors

[lovincyrus]

This PR implements a shared, config-driven multi-step form architecture for object storage connectors (GCS, S3, Azure). It replaces a GCS-specific implementation with a generalized system that splits connector configuration into two steps:

1. Connector step: Authentication configuration
2. Source step: Resource path and model creation

• [x] Azure explicit credentials
• [x] S3 explicit credentials
• [x] Remove Skip and add a contextual messaging in right panel that acts the same
• [x] Add public option that changes button to Continue
• [x] Pin Help text to bottom of right panel, update messaging on Source model page
• [x] Change Test and Import Data to, Just Import Data or Create Model
• [x] https://github.com/rilldata/rill/pull/8395/files
• [x] Connector validation for authentication method
• [x] ⚠️ Centralize auth method state in connectorStepStore to avoid initialization races
• [x] ⚠️ Extract isSubmitDisabled logic to testable functions
• [x] ⚠️ Add constants for magic strings ("public", etc.)

Checklist:

• [x] Covered by tests
• [x] Ran it and it works as intended
• [x] Reviewed the diff before requesting a review
• [ ] Checked for unhandled edge cases
• [x] Linked the issues it closes
• [ ] Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• [ ] Intend to cherry-pick into the release branch
• [x] I'm proud of this work!