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 adds support for using database credentials from the credentials file when verifying unmanaged tablet configurations. Previously, unmanaged tablets required explicit password specification, but now they can fall back to using credentials from the configured credentials server.

• Enhances unmanaged tablet configuration verification to check credentials server when password is not explicitly provided
• Adds test helper function to allow setting credentials file path for testing purposes
• Includes test coverage for the new credential fallback functionality

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
go/vt/vttablet/tabletserver/tabletenv/config.go	Modified verification logic to attempt credential lookup before failing
go/vt/dbconfigs/credentials.go	Added test helper function for setting credentials file path
go/vt/vttablet/tabletserver/tabletenv/config_test.go	Added test case for credential fallback functionality
go/vt/vttablet/tabletserver/tabletenv/data/db_credentials.json	Added test credentials file for unit testing

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[Copilot]

The function comment should specify what the dbCredentialsFile variable represents and mention that this modifies global state, which could affect other tests running concurrently.