fix: make a Datasphere describe that cannot answer fail instead of answering

[jrosskopf]

Closes #244.

The defect

LoadResourceDetails turned every failure into the row {"error", "<message>"}, which was then resize(15)'d and emitted. So:

SELECT name FROM datasphere_describe_asset('SP', 'MISSING');

answered with the literal string "error" as the asset's name. A caller could not tell a missing asset from an asset called error, nor an unreachable tenant from an empty one — and nothing in the result said anything had gone wrong.

The reported disagreement, settled

The two reviewers disagreed about whether a missing asset throws on a STRUCT cast or emits an error row. Reading the code decides it, and both were partly right:

• The extended-metadata block resize(15)s before the scan inspects the row, so the row wins — no cast error.
• But the cast concern was real and latent: resize() fills with a default-constructed Value (untyped NULL), and column 10 is declared a STRUCT. Only the catch branch filled it with MakeEmptyAnalyticalSchemaValue(); the success branch didn't. Same column, guarded in one branch of two.

Both now fill it, and the root cause — an outer catch (const std::exception&) that swallowed everything, including the explicit throws added here — propagates instead.

The scan's two fallbacks are gone: one wrote the same VARCHAR into all 15 columns including the STRUCT, the other reported "Error: No space found" as the space's name.

What removing the catch-all exposed

An incomplete fix of my own from #243. The describe binds parsed secret := and used it for their own token fetch, but never assigned it to the bind data — so LoadResourceDetails, which fetches a token of its own, kept using the "datasphere" default.

The test asserting that threading had been passing because the resulting failure was swallowed. Assigned now, and that test red-checks properly. Worth stating plainly: a swallowing catch had been hiding a bug in the fix for a different bug.

Testing

Against a tenant that does not resolve, which needs no real credentials — the catalog URLs are built as https://<tenant>.<data_center>.hcs.cloud.sap/... with no loopback hatch, so a local server can't stand in. The fixture carries a pre-valid token so resolution never reaches RefreshTokens and cannot open a browser.

• 3117 assertions in 580 test cases
• Red-checked both ways: restoring the catch-all fails the describe cases; removing the secret_name assignment fails the threading case

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