fix: avoid duplicate keysign for outTx already pending

[ws4charlie]

Description

We should skip unnecessary keysign if the tx being signed is already pending.

Closes: 1478

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Include instructions and any relevant details so others can reproduce.

• Tested CCTX in localnet
• Tested in development environment
• Go unit tests
• Go integration tests
• Tested via GitHub Actions

Checklist:

• I have added unit tests that prove my fix feature works

[github-actions]

!!!WARNING!!!
nosec detected in the following files: zetaclient/zetacore_observer.go

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[lumtis]

The logic looks ok, left few comments