fix(x/staking): handle redelegation when unbonded source is removed (backport #26408)

[mergify]

Summary

• handle ErrNoValidatorFound in staking redelegation begin-info as an immediate-completion path when the source validator was already unbonded and removed after its final shares were consumed
• add a keeper-level regression test covering 100% redelegation from an unbonded source validator
• add an app-level no-mocks regression test reproducing the same flow through full message execution

Test plan

• go test ./x/staking/keeper -run TestRedelegateAllSharesFromUnbondedValidator
• go test ./x/staking -run TestBeginRedelegateAllSharesFromUnbondedSourceNoMocks

Made with Cursor

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This is an automatic backport of pull request #26408 done by Mergify.

[mergify]

Cherry-pick of 1a79e79 has failed:

On branch mergify/bp/release/v0.53.x/pr-26408
Your branch is up to date with 'origin/release/v0.53.x'.

You are currently cherry-picking commit 1a79e7960.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
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Changes to be committed:
	modified:   x/staking/app_test.go
	modified:   x/staking/keeper/delegation.go
	modified:   x/staking/keeper/delegation_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[greptile-apps]

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