fix(cli): report correct entity type for missing property

[jx2lee]

Summary

• properties get always reported "on namespace" in its error message even when the target was a table, since the message was hardcoded instead of using the actual entity type.
• Switched to fmt.Errorf with get.Type so the message reflects "namespace" or "table" correctly.
• Add unit test verifying properties get reports correct entity type on missing property

Test plan

go test ./cmd/iceberg/

Ran against a local hadoop catalog, requesting a non-existent property:

Before

$ iceberg properties get table ns1.t1 nonexistent_prop
could not find property nonexistent_prop on namespace ns1.t1   # wrong: says "namespace" for a table

After

$ iceberg properties get table ns1.t1 nonexistent_prop
could not find property nonexistent_prop on table ns1.t1       # correct

$ iceberg properties get namespace ns1 nonexistent_prop
could not find property nonexistent_prop on namespace ns1      # unchanged, still correctn	

[tanmayrauth]

Fix is correct — validateEntityType guarantees get.Type is only ever "namespace" or "table", so the interpolated message is safe. Couple of nits inline.

[tanmayrauth]

The test name TestRunPropertiesGetNamespaceNotFound로 has a stray Korean character 로 on the end — looks like an accidental keystroke. It compiles and runs (Go allows Unicode identifiers), but it shows up as garbage in CI logs and when grepping test names, and it's inconsistent with the sibling TestRunPropertiesGetTableNotFound. Rename to TestRunPropertiesGetNamespaceNotFound.

[tanmayrauth]

The return here is unreachable — osExit is os.Exit in prod (never returns) and panics in tests, and this is already the end of the case. Harmless, but you can drop it.