fix: ensure GeneratePassword returns password with mixed cases

[yipeng1030]

fix #8837

Problem: Mixed-case password doesn't guarantee having both uppercase and lowercase.
Fix: Introduce EnsureMixedCase() post-processor to guarantee at least one uppercase and one lowercase.
Testing: Verified consistent generation with same seed, plus enforced mixed-case distribution.

[codecov]

Codecov Report

Attention: Patch coverage is 78.00000% with 11 lines in your changes missing coverage. Please review.

Project coverage is 60.18%. Comparing base (a5bcd50) to head (92f2889).
Report is 99 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/common/password.go	78.00%	7 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8904      +/-   ##
==========================================
- Coverage   60.28%   60.18%   -0.10%     
==========================================
  Files         376      387      +11     
  Lines       45858    46865    +1007     
==========================================
+ Hits        27645    28208     +563     
- Misses      15614    15958     +344     
- Partials     2599     2699     +100     

Flag	Coverage Δ
unittests	60.18% <78.00%> (-0.10%)	⬇️

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[yipeng1030]

/cherry-pick release-1.0-beta

[apecloud-bot]

🤖 says: cherry pick action finished successfully 🎉!
See: https://github.com/apecloud/kubeblocks/actions/runs/13240140750

[yipeng1030]

/cherry-pick release-0.9

[apecloud-bot]

🤖 says: Error cherry-picking.

CONFLICT (modify/delete): controllers/apps/cluster/transformer_cluster_sharding_account.go deleted in HEAD and modified in 972c0d8 (fix: ensure GeneratePassword returns password with mixed cases (#8904)). Version 972c0d8 (fix: ensure GeneratePassword returns password with mixed cases (#8904)) of controllers/apps/cluster/transformer_cluster_sharding_account.go left in tree.
CONFLICT (modify/delete): controllers/apps/component/transformer_component_account.go deleted in HEAD and modified in 972c0d8 (fix: ensure GeneratePassword returns password with mixed cases (#8904)). Version 972c0d8 (fix: ensure GeneratePassword returns password with mixed cases (#8904)) of controllers/apps/component/transformer_component_account.go left in tree.
Auto-merging pkg/common/password.go
Auto-merging pkg/common/password_test.go
error: could not apply 972c0d8... fix: ensure GeneratePassword returns password with mixed cases (#8904)
hint: After resolving the conflicts, mark them with
hint: "git add/rm ", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config advice.mergeConflict false"

[apecloud-bot]

🤖 says: ‼️ cherry pick action failed.
See: https://github.com/apecloud/kubeblocks/actions/runs/13302963925