tici: fix lost common prefix when apply config

[3pointer]

Summary

Fix TiCI config generation to correctly include s3.prefix for both meta and worker components. Previously, the generated TOML contained a prefix = "%s" placeholder but the fmt.Sprintf arguments did not include tc.Spec.TiCI.S3.Prefix, which could lead to missing/invalid s3.prefix in the rendered config. 

This PR also adds a unit test to ensure s3.prefix is present and the generated config is valid TOML.

What’s Changed

Code changes

•	Include s3.Prefix in config rendering
•	Update the fmt.Sprintf argument lists so the generated TOML uses tc.Spec.TiCI.S3.Prefix for the prefix = "%s" field in:
•	buildTiCIMetaConfig(...)
•	buildTiCIWorkerConfig(...)  

Tests

•	Add TestBuildTiCIConfigIncludesS3Prefix

[ti-chi-bot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: csuzhangxc

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [csuzhangxc]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-02-27 07:41:21.040642075 +0000 UTC m=+427153.555436684: ☑️ agreed by csuzhangxc.