Bump Golang to v1.26.5

[FengyuanYin]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Upgrades the project's Go version from 1.26.4 to 1.26.5.

This change updates the Go version declared in go.mod and synchronizes the Go builder images used by all Dockerfiles. GitHub Actions workflows read the version directly from go.mod, so no additional CI changes are required.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

The full test suite could not be completed locally because dependencies could not be downloaded from proxy.golang.org. The installed Go toolchain is Go 1.26.5, and all Go version references have been updated consistently.

Does this PR introduce a user-facing change?:

NONE

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[volcano-sh-bot]

Welcome @SipengShen01! It looks like this is your first PR to volcano-sh/agentcube 🎉

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.98%. Comparing base (3b19390) to head (5903457).
⚠️ Report is 34 commits behind head on main.
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #452      +/-   ##
==========================================
+ Coverage   58.41%   59.98%   +1.57%     
==========================================
  Files          36       36              
  Lines        3463     3589     +126     
==========================================
+ Hits         2023     2153     +130     
+ Misses       1231     1216      -15     
- Partials      209      220      +11     

Flag	Coverage Δ
unittests	59.98% <ø> (+1.57%)	⬆️

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[RainbowMango]

/lgtm
/approve

[volcano-sh-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: RainbowMango

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