fix: allow merge insert write parameters

[zlepper]

Fixes #6426.

Merge-insert currently creates data fragments with WriteParams::default(), so callers cannot configure blob handling or any other write behavior.

Add MergeInsertBuilder::with_write_params, which accepts a callback over the current parameters and forwards the resulting configuration through both full-schema merge fragment-write paths.

The regression test verifies that merge-insert forwards allow_external_blob_outside_bases for an external Blob V2 URI.

Do note: I'm not entirely sure about the target branch here, but copilot figured this was the correct target to get these changes in a minor release :)

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