Fix LaTeX template engine options to emit valid latexmk flags

[Copilot]

The LaTeX devcontainer template exposed latexEngine values (xetex/luatex/pdftex) that were injected directly into latexmk as -<value>, producing invalid flags (notably -xetex). This updates template engine values to align with valid latexmk arguments and keeps package installation behavior consistent.

• Template option normalization

  • Updated latexEngine enum in templates/src/latex/devcontainer-template.json to:
    • pdflatex, xelatex, lualatex, latex
  • Updated default to xelatex
  • Updated template metadata text/keywords to match supported engine names.

• Runtime package-install compatibility

  • Updated postStartCommand in templates/src/latex/.devcontainer.json to map engine names to TeX Live package names before tlmgr install:
    • xelatex -> xetex
    • lualatex -> luatex
    • pdflatex -> pdftex
    • latex -> latex

• Template-facing docs and regression coverage

  • Updated templates/src/latex/README.md engine wording/default to match template options.
  • Extended templates/test/latex/test.sh with explicit checks for supported latexmk engine flags.

"latexEngine": {
  "type": "string",
  "enum": ["pdflatex", "xelatex", "lualatex", "latex"],
  "default": "xelatex"
}