client: expose S3-compatible endpoint and region outputs

[goanpeca]

Summary

Expose non-secret S3-compatible connection metadata derived from SDK authorization so downstream workflow steps can configure S3-compatible tooling without re-authorizing.

Why

The SDK includes an S3 helper that derives endpoint, region, and path-style configuration from B2 auth state. The action already authorizes, but it does not emit the S3 endpoint or region. Workflows that use AWS CLI, rclone, or custom S3 clients after a B2-native action step must rediscover or hard-code those values.

SDK support (already available)

• createS3ClientConfig(...) from @backblaze-labs/b2-sdk/s3
• StorageApiInfo.s3ApiUrl
• S3 region extraction from the S3 API URL

Proposed surface

New outputs after authorization:

• s3-endpoint
• s3-region
• s3-force-path-style (true)

Do not output the application key secret. Downstream tools should continue to read credentials from existing secrets/env vars.

Acceptance criteria

• The action emits s3-endpoint and s3-region derived from the SDK/auth state.
• No secret credential material is emitted in outputs or logs.
• Outputs are available for every action verb after successful authorization.
• Documentation shows a downstream S3-compatible CLI/client example using existing secrets plus the new non-secret outputs.
• Tests cover region extraction and secret non-disclosure.

Out of scope

• Implementing S3 operations directly in this action.
• Emitting AWS-style secret/access-key config as a single JSON blob.