playground: Fix tiflash start with old mode unexpectedly

[JaySon-Huang]

What problem does this PR solve?

When start playground with specified tiflash binary using --tiflash.binpath, tiflash will start using the config that is only for version earlier than v7.1. That is unexpected.

./bin/tiup-playground nightly --mode 'tidb-fts' --tici.binpath /DATA/disk1/jaysonhuang/tici/target/debug/tici-server --s3.endpoint http://10.2.12.79:9000 --s3.bucket ticidefault --tiflash.binpath /DATA/disk1/jaysonhuang/tiflash/cmake-build-debug/dbms/src/Server/tiflash --tag jayson-play-tici

What is changed and how it works?

When start playground with specified tiflash binary, the inst.Version is empty. Usually this case is use for testing or development, start with new mode.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

# Run the command below and check the generated tiflash config
./bin/tiup-playground nightly --mode 'tidb-fts' --tici.binpath /DATA/disk1/jaysonhuang/tici/target/debug/tici-server --s3.endpoint http://10.2.12.79:9000 --s3.bucket ticidefault --tiflash.binpath /DATA/disk1/jaysonhuang/tiflash/cmake-build-debug/dbms/src/Server/tiflash --tag jayson-play-tici

• 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:

Fixed a bug in playground that caused TiFlash to launch with unexpected configurations when an explicit binary path was specified

[ti-chi-bot]

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 41.51%. Comparing base (0890ea9) to head (41d09cf).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2670      +/-   ##
==========================================
- Coverage   41.52%   41.51%   -0.01%     
==========================================
  Files         417      417              
  Lines       45579    45580       +1     
==========================================
- Hits        18926    18922       -4     
- Misses      24109    24112       +3     
- Partials     2544     2546       +2     

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-01-14 08:00:13.192786029 +0000 UTC m=+430857.254650940: ☑️ agreed by xhebox.