fix(index): keep the requested index name after a failed uncommitted build

[LuciferYang]

Problem

execute_uncommitted takes the name out of the builder before it rejects a duplicate, so a build that fails there leaves self.name empty. A caller that holds the builder and retries then falls into default-name generation and commits under <column>_idx instead of the name it asked for. The method is pub on &mut self, so holding it across a retry is a supported shape.

Fixes #9010.

What this changes

Read the name without consuming it. The same take() in execute_multi_segment_fmindex is unobservable, since its only caller consumes the builder, but it is changed for consistency.

One behaviour change worth naming: a second execute_uncommitted on a builder whose first call already committed under that name now returns Index name 'X' already exists where before it quietly succeeded under <column>_idx. Nothing in the repo does that, and silently renaming the caller's index is the bug being fixed, but it is a new error on a path that used to succeed.

Test plan

test_failed_execute_uncommitted_preserves_name builds retry_idx, then runs execute_uncommitted on a held builder against the same name so it fails on the duplicate, and asserts the name survived. It then does the retry a caller would do and asserts the dataset still has exactly one index.

Restoring the take() fails both assertions independently: the field reads None, and with that assertion removed the retry commits a second index, so the last check reports ["retry_idx", "text_idx"] against ["retry_idx"].

• cargo test -p lance --lib 3381 passed, 3 ignored
• cargo test -p lance --lib index::create:: 43 passed
• cargo clippy --all --tests --benches -- -D warnings clean
• cargo fmt --all --check clean

[lance-gatekeeper]

✅ Gate recommendation: approve.

The change fixes the builder-state leak at its source: a failed uncommitted build retains the explicitly requested index name, so a retry cannot silently fall back to a generated name. Copying the small name value before fallible work also keeps the error paths simple and does not alter index storage.