fix: support quoted structure names in structure command diagnostics

[Copilot]

Quoted structure names in commands (e.g. structure load "mystructure:house" ~ ~ ~) triggered false "missing structure" errors because the diagnoser compared the raw token—including surrounding quotes—against stored unquoted IDs.

Changes

• diagnose_structure_implementation — call Text.UnQuote() on the resolved ID string before all lookup checks, consistent with how other diagnosers (tickingarea, fake-entity, mcfunction, etc.) handle quoted tokens.
• Structure diagnoser test — add a case asserting that a quoted token resolves cleanly when the unquoted ID exists in the pack.

# Before — false positive error
structure load "mystructure:house" ~ ~ ~
#              ^^^^^^^^^^^^^^^^^^^  → looked up as `"mystructure:house"`, not found

# After — no error
structure load "mystructure:house" ~ ~ ~
#                                  → stripped to `mystructure:house`, found ✓