fix: use typed NamespaceError::TableNotFound in DirectoryNamespace instead of plain strings

[LuciferYang]

Summary

Replace Error::namespace_source(String) with NamespaceError::TableNotFound in dir.rs and dir/manifest.rs so that the JNI bridge can produce typed Java TableNotFoundException instead of raw RuntimeException.

Problem

When a table doesn't exist, dir.rs and dir/manifest.rs construct errors like:

Error::namespace_source(format!("Table does not exist: {}", table_name).into())

This wraps a plain String as Box<dyn Error>. The JNI bridge (java/lance-jni/src/error.rs) tries to downcast the source to NamespaceError to throw a typed LanceNamespaceException(TABLE_NOT_FOUND), but the downcast fails on a String and falls back to RuntimeException.

The full typed error plumbing already exists — NamespaceError::TableNotFound → lance_core::Error::Namespace (preserves the NamespaceError) → JNI downcast → Java TableNotFoundException. Only the error construction at the call sites was missing it.

This causes issues in the Spark connector (lance-format/lance-spark#217) where Spark 4.0+ expects NoSuchTableException from loadTable() but gets RuntimeException, making tableExists() crash instead of returning false.

Fix

• dir.rs: 3 occurrences — "Table does not exist: {table_name}" → NamespaceError::TableNotFound { message: table_name }
• dir/manifest.rs: 4 occurrences — "Table '{name}' not found" → NamespaceError::TableNotFound { message: name }
• Updated 2 test assertions in dir.rs to match the new display format ("Table not found: ...")

Test plan

• test_describe_nonexistent_table — passes with updated assertion
• test_table_exists — passes with updated assertion
• test_table_exists_fails_for_deregistered_v1 — passes (unchanged)
• test_convert_to_lance_error in lance-namespace — confirms downcast chain works
• cargo check -p lance-namespace-impls — compiles cleanly

[github-actions]

Review

Clean, well-scoped bugfix. The error conversion chain (NamespaceError::TableNotFound → lance_core::Error::Namespace → JNI downcast) is correctly wired and already tested in test_convert_to_lance_error. No issues found.

LGTM.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.12195% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-namespace-impls/src/dir.rs	96.87%	1 Missing ⚠️
rust/lance-namespace-impls/src/dir/manifest.rs	88.88%	1 Missing ⚠️

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