fix: validate transform parameters in metadata

[fallintoplace]

Summary

• add Transform::Validate to reuse bind-time transform checks during metadata validation
• reject invalid bucket/truncate parameters when parsing transform strings
• make partition specs and sort orders reject non-positive bucket/truncate parameters during schema-bound validation

Validation

• cmake -S . -B build-transform-params -G Ninja -DICEBERG_BUILD_BUNDLE=OFF -DICEBERG_BUILD_REST=OFF -DICEBERG_BUILD_SHARED=OFF
• ctest --test-dir build-transform-params -R schema_test --output-on-failure

[huan233usc]

LGTM. Correct and well-targeted: metadata validation now reaches the bind-time bucket/truncate parameter checks via Validate→Bind→Make, and the new tests genuinely exercise the parameter path (source types are valid for their transforms). Two minor, non-blocking nits inline.

[huan233usc]

Worth a one-line comment on why both CanTransform and Bind are called here: CanTransform keeps type mismatches as InvalidArgument/"Invalid source type", whereas Bind→Make would report them as NotSupported. Otherwise a future cleanup may drop CanTransform as redundant and silently change the error kind.

[huan233usc]

The bucket/truncate positivity rule (and its message) is now duplicated here and in BucketTransform::Make/TruncateTransform::Make. Fine for fail-fast on unbound transforms, but easy to drift — consider a shared helper if it grows.