chore: update Go toolchain to 1.26.2

[ranxi2001]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Update the AgentCube Go toolchain baseline to Go 1.26.2 in a clean branch based on the original upstream main branch. This keeps the Go/toolchain prerequisite separate from the agent-sandbox dependency adaptation work, so the original project is verified on the new Go baseline before the dependent feature branch rebases onto it.

This changes only Go/toolchain references:

• go.mod Go version
• GitHub Actions Go versions for build/codegen/e2e/lint/coverage
• Docker builder images for workloadmanager, router, and picod

Which issue(s) this PR fixes:
Refs volcano-sh#386

Special notes for your reviewer:

• Scope: fork pre-review/staging PR before preparing an upstream PR.
• Upstream target: no upstream PR has been opened yet. After this fork PR is reviewed, a separate upstream PR body will be prepared for approval before posting to volcano-sh/agentcube.
• Fork CI result: all checks passed on this PR, including codespell, Codegen Check, Python Lint, build, coverage, e2e-test, golangci-lint, and python-sdk-tests.
• Tests run locally:
  • go list ./... | grep -v '^github.com/volcano-sh/agentcube/test/e2e$' | xargs go test -count=1
  • go test -race -v -coverprofile=coverage.out -coverpkg=./pkg/... ./pkg/...
  • make build-all
  • make lint
  • make gen-check
  • docker build -f docker/Dockerfile -t agentcube-go126-workloadmanager:test .
  • docker build -f docker/Dockerfile.router -t agentcube-go126-router:test .
  • docker build -f docker/Dockerfile.picod -t agentcube-go126-picod:test .
• AI assistance: Used Codex to inspect toolchain references, run validation commands, and prepare this fork staging PR. I reviewed and validated the changes.

Does this PR introduce a user-facing change?:

NONE

[ranxi2001]

Superseded by #3, which updates the target to Go 1.26.4 and centralizes GitHub Actions Go version management with go-version-file: go.mod.