chore(deps): upgrade Go from 1.25.2 to 1.26

[zhouzhuojie]

Summary

Upgrade Go version from 1.25.2 to 1.26.

Changes

• go.mod: go 1.25.2 → go 1.26
• Dockerfile: golang:1.25.2-alpine → golang:1.26-alpine, remove GOEXPERIMENT=greenteagc
• integration_tests/Dockerfile-Integration-Test: golang:1.25.2-alpine → golang:1.26-alpine, remove GOEXPERIMENT=greenteagc
• .github/workflows/ci.yml: go-version: 1.25.2 → go-version: 1.26, remove GOEXPERIMENT=greenteagc

Verification

• ✅ Tests pass: go test ./pkg/...
• ✅ Build succeeds: go build
• ✅ Lint passes: golangci-lint (no issues)

Go 1.26 Highlights

• ✅ Green Tea GC is now the default (no longer needs GOEXPERIMENT)
• new() function now accepts expressions as operands
• Generic types can self-reference in type parameter lists
• Revamped go fix with modernizers for automatic code updates