Restore super_read_only to initial state after SetReplicationSource

[tanjinx]

After a PRIMARY is demoted, DemotePrimary sets super_read_only=ON for safety during the critical demotion phase. However, normal replicas don't have super_read_only enabled by default (unless explicitly configured in my.cnf).

This change captures the initial super_read_only state at tablet startup (reflecting the my.cnf configuration) and restores it after SetReplicationSource completes. This ensures demoted primaries behave consistently with other replicas in the shard.

Files Changed:

1. go/vt/vttablet/tabletmanager/tm_init.go
   - Added disableSuperReadOnly flag variable (default false)
   - Registered --disable_super_read_only flag
2. go/vt/vttablet/tabletmanager/rpc_replication.go
   - Added logic to turn off super_read_only after demotion when flag is enabled
   - Only applies when wasPrimary && disableSuperReadOnly
3. go/flags/endtoend/vttablet.txt
   - Added documentation for --disable_super_read_only flag
4. go/flags/endtoend/vtcombo.txt
   - Added documentation for --disable_super_read_only flag

Behavior:

Default (--disable_super_read_only=false):

• After PRIMARY demotion, super_read_only remains ON
• Provides extra protection against accidental writes
• Demoted primary has different state than normal replicas

Enabled (--disable_super_read_only=true):

• After PRIMARY demotion, super_read_only is turned OFF
• Demoted primary matches normal replica behavior
• Consistent with default MySQL configuration

The safety benefit of super_read_only=ON during the critical demotion phase is preserved in
both cases.

Description

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

AI Disclosure

[codecov-commenter]

Codecov Report

❌ Patch coverage is 10.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.78%. Comparing base (b1ab347) to head (2b9df49).
⚠️ Report is 32 commits behind head on slack-22.0.

Files with missing lines	Patch %	Lines
go/vt/vttablet/tabletmanager/rpc_replication.go	0.00%	9 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           slack-22.0     #801      +/-   ##
==============================================
+ Coverage       67.53%   69.78%   +2.25%     
==============================================
  Files            1600     1605       +5     
  Lines          261782   214036   -47746     
==============================================
- Hits           176786   149374   -27412     
+ Misses          84996    64662   -20334     

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