fix: namespace_exists/table_exists raise NamespaceNotFoundError/TableNotFoundError

[lilei1128]

namespace_exists() and table_exists() were raising RuntimeError instead of
the documented NamespaceNotFoundError / TableNotFoundError because the
implementations in dir.rs and manifest.rs used Error::namespace_source(String)
— a plain boxed string — which cannot be downcast to NamespaceError in
infer_error(), triggering the fallback RuntimeError path.

Fix by constructing proper NamespaceError variants:

• manifest.rs namespace_exists: NamespaceError::NamespaceNotFound
• manifest.rs table_exists: NamespaceError::TableNotFound
• dir.rs namespace_exists: NamespaceError::Unsupported (child NS without manifest)
• dir.rs table_exists: NamespaceError::TableNotFound

Also fix rest_adapter.rs error_to_response to dispatch on NamespaceError
error codes instead of fragile string matching, and rest.rs HTTP helpers to
parse the numeric error code from the response body and reconstruct the
correct NamespaceError variant, so the full REST round-trip also raises the
right Python exception.

Update tests to assert TableNotFoundError / NamespaceNotFoundError specifically.

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[lilei1128]

close #6240

[wjones127]

This PR looks good. I have a few non-blocking suggestions, but those are pre-existing issues so I'd be fine addressing them in a different PR.

[wjones127]

suggestion: This message duplicates what is in the Display implementation for this error:

lance/rust/lance-namespace/src/error.rs

Lines 180 to 182 in baa13ab

	/// The specified namespace does not exist.
	#[snafu(display("Namespace not found: {message}"))]
	NamespaceNotFound { message: String },

The error will print as:

Namespace not found: Namespace '{name}' not found

Consider just putting the quoted object id as the message:

Suggested change

	Err(NamespaceError::NamespaceNotFound {
	message: format!("Namespace '{}' not found", object_id),
	}
	.into())
	Err(NamespaceError::NamespaceNotFound {
	message: format!("'{}'", object_id)
	}
	.into())

Then the message would be like:

Namespace not found: '{name}'

[lilei1128]

Thank you for your correction.

[wjones127]

suggestion: similar to namespace error comment, this message duplicates the same text. Consider shortening it to:

Suggested change

	return Err(NamespaceError::TableNotFound {
	message: format!("Table does not exist: {}", table_name),
	}
	.into());
	return Err(NamespaceError::TableNotFound {
	message: format!("'{}'", table_name),
	}
	.into());

[wjones127]

praise: I'm very happy to see us asserting more specific errors here 😄

[lilei1128]

Thanks! Happy to help make our error handling clearer 😊

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-namespace-impls/src/dir.rs	71.42%	0 Missing and 2 partials ⚠️

📢 Thoughts on this report? Let us know!

[wjones127]

I've received a request from @hamersaw to merge #6275 in favor of this PR, as it contains these changes plus some more. I'll close this out, but thank you for working on this. It was reviewing this I was able to see the duplicate info in the errors more easily 🙌