support TiFlash startup when no TiDB server exists

[kolafish]

What problem does this PR solve?

when start TiCI without TiDB server

tiup playground:v1.16.2-feature.fts --mode tidb-fts --s3.bucket bucket-jane --tiflash 1 --db 0 

TiFlash server can not be started.
This pr support to start TiFlash server without TiDB server

close #xxx

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

NONE

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[LGTM Timeline notifier]

Timeline:

• 2026-01-05 03:34:01.599382226 +0000 UTC m=+586797.417690648: ☑️ agreed by xhebox.