Harden the pyvolca converter example

[ccomb]

Summary

• copy the source into the temporary run directory and declare it in volca.toml instead of uploading it over a raceable loopback port
• remove explicit unload/delete cleanup that could mask the primary conversion failure; server shutdown and temporary-directory cleanup now own the lifecycle
• narrow the pyvolca 0.8.0 HTTP 406 fallback and retain warning propagation
• add behavioral coverage for fallback matching and warnings

Verification

• uv run --with-editable . --with pytest pytest -q — 226 passed, 7 skipped
• uv run --with-editable . --with pyright==1.1.411 pyright — clean
• real pyvolca 0.8.0 / VoLCA 0.9.1 run produced valid ILCD, EcoSpold 1, and EcoSpold 2 outputs