Allow the use of db credentials when verifying unmanaged tablets conf…

[mhamza15]

…ig (vitessio#17984)

This is a backport of vitessio#17984

Description

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[Copilot]

Pull Request Overview

This PR enables the use of database credentials from a credentials file when verifying unmanaged tablet configurations. It backports functionality that allows tablets to fall back to credential files when passwords are not explicitly provided in configuration.

• Modified the unmanaged tablet configuration verification to check for credentials via GetCredentialsServer() when password is empty
• Added a test utility function to set credential file paths for testing purposes
• Added test coverage for the new credential fallback functionality

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
go/vt/vttablet/tabletserver/tabletenv/config.go	Updated verification logic to fetch passwords from credentials server when not provided
go/vt/dbconfigs/credentials.go	Added test utility function to set credential file path
go/vt/vttablet/tabletserver/tabletenv/config_test.go	Added test case for credential file fallback behavior
go/vt/vttablet/tabletserver/tabletenv/data/db_credentials.json	Added test credential file with sample user/password

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