dep: update build with llgo v0.12.1

[luoliwoshang]

[luoliwoshang]

/code

[fennoai]

Completed: Update build with llgo v0.12.1

PR created: #646

Changes

The go.mod already referenced llgo v0.12.1 (via #624), but CI pipelines and documentation still used v0.12.0. Updated the following files to align everything to v0.12.1:

File	Change
.github/workflows/go.yml	llgo: [v0.12.0] → [v0.12.1]
.github/workflows/gentest.yml	llgo: [v0.12.0] → [v0.12.1]
.github/workflows/end2end.yml	llgo: [v0.12.0] → [v0.12.1]
.github/actions/setup-llcppg/action.yml	Default v0.12.0 → v0.12.1
.github/copilot-instructions.md	Commit hash updated to v0.12.1 tag (298669b)

Build validated locally with go build -v ./....