Update gemini image to 0.51.0

[kelos-bot]

/kind feature

What type of PR is this?

Feature

What this PR does / why we need it:

Updates the pinned GEMINI_CLI_VERSION in gemini/Dockerfile from 0.50.0 to 0.51.0.

Which issue(s) this PR is related to:

N/A

Special notes for your reviewer:

Package page: https://www.npmjs.com/package/@google/gemini-cli

Does this PR introduce a user-facing change?

Yes

Update gemini image from 0.50.0 to 0.51.0

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Summary by cubic

Update the gemini Docker image to use @google/gemini-cli 0.51.0 (was 0.50.0) by bumping GEMINI_CLI_VERSION in gemini/Dockerfile. This pulls in the latest CLI fixes and improvements.

^{Written for commit 208c7b5. Summary will update on new commits.}